diff --git a/.github/CONTRIBUTING.md b/.github/CONTRIBUTING.md
index 9594796..4fd72f1 100644
--- a/.github/CONTRIBUTING.md
+++ b/.github/CONTRIBUTING.md
@@ -116,7 +116,7 @@ Licensing
 ---------
 
 It should also be made clear that **all code contributed to yii2-helpers** must be
-licensable under the [BSD-3 license][licensing].  Code that cannot be released
+licensable under the [BSD-3-Clause license][licensing].  Code that cannot be released
 under this license **cannot be accepted** into the project.
 
 [issue-search]: https://github.com/kartik-v/yii2-helpers/search?q=&type=Issues
diff --git a/CHANGE.md b/CHANGE.md
index 6cc15d4..0397801 100644
--- a/CHANGE.md
+++ b/CHANGE.md
@@ -1,15 +1,28 @@
 Change Log: `yii2-helpers`
 ==========================
 
+## Version 1.3.7
+
+**Date:** 26-Sep-2018
+
+- Enhancements to support Bootstrap v4.x.
+- Move all source code to `src` directory.
+- (enh #44): Secure `userIP` function.
+- (enh #43): Update Vietnamese Translations.
+- (enh #42): Update Hungarian Translations.
+- (enh #41): Update German Translations.
+- (enh #40): Fixed `a moment ago` readout.
+- (enh #39): Update Greek Translations.
+
 ## Version 1.3.6
 
 **Date:** 26-Nov-2016
 
-- (enh #33): Update Italian Translations.
-- (enh #35): Add Slovak Translations.
-- (enh #36): Update Russian Translations.
-- (enh #37): Update Finnish Translations.
 - (bug #38): Fix buttonGroup `item` parsing when set as Closure.
+- (enh #37): Update Finnish Translations.
+- (enh #36): Update Russian Translations.
+- (enh #35): Add Slovak Translations.
+- (enh #33): Update Italian Translations.
 - Update message config to include all default standard translation files.
 - Add github contribution and issue/PR logging templates.
 - Add branch alias for dev-master latest release.
@@ -19,9 +32,9 @@ Change Log: `yii2-helpers`
 
 **Date:** 12-Dec-2015
 
-- (enh #27): Add Ukranian Translations.
-- (enh #28): Add Italian Translations.
 - (enh #29): Code style and formatting enhancements.
+- (enh #28): Add Italian Translations.
+- (enh #27): Add Ukranian Translations.
 
 ## Version 1.3.3
 
@@ -44,23 +57,23 @@ Change Log: `yii2-helpers`
 
 **Date:** 15-Jun-2015
 
-- (enh #15): Initialize i18N for getBrowser method.
-- (enh #16): Add Russian translation messages.
-- (enh #17, #18): Update Russian translation messages.
-- (enh #19): Add Portuguese Brazilian translation messages.
-- (bug #21): Remove duplicate browsers in list.
 - (enh #22): Add Spanish translations.
+- (bug #21): Remove duplicate browsers in list.
+- (enh #19): Add Portuguese Brazilian translation messages.
+- (enh #17, #18): Update Russian translation messages.
+- (enh #16): Add Russian translation messages.
+- (enh #15): Initialize i18N for getBrowser method.
 
 ## Version 1.3.0
 
 **Date:** 23-Jan-2015
 
-- (enh #7): Implement translations.
-- (enh #8): Convert days, months, ones, tens, triplets to static methods
-- (enh #9): Add/Enhance new methods for `dayList`, `monthList` and `dateList`.
-- (enh #10): Enhance `timeList` method to include ranges and intervals.
-- (enh #11): Add French Translations and fix `genCalList`.
 - (enh #12): Completed French Translations.
+- (enh #11): Add French Translations and fix `genCalList`.
+- (enh #10): Enhance `timeList` method to include ranges and intervals.
+- (enh #9): Add/Enhance new methods for `dayList`, `monthList` and `dateList`.
+- (enh #8): Convert days, months, ones, tens, triplets to static methods
+- (enh #7): Implement translations.
 
 ## Version 1.2.0
 
diff --git a/README.md b/README.md
index d959c00..2d969f7 100644
--- a/README.md
+++ b/README.md
@@ -1,7 +1,8 @@
 yii2-helpers
 ============
 
-[![Latest Stable Version](https://img.shields.io/packagist/v/kartik-v/yii2-helpers.svg)](https://packagist.org/packages/kartik-v/yii2-helpers)
+[![Latest Stable Version](https://poser.pugx.org/kartik-v/yii2-helpers/v/stable)](https://packagist.org/packages/kartik-v/yii2-helpers)
+[![Latest Unstable Version](https://poser.pugx.org/kartik-v/yii2-helpers/v/unstable)](https://packagist.org/packages/kartik-v/yii2-helpers)
 [![License](https://poser.pugx.org/kartik-v/yii2-helpers/license)](https://packagist.org/packages/kartik-v/yii2-helpers)
 [![Total Downloads](https://poser.pugx.org/kartik-v/yii2-helpers/downloads)](https://packagist.org/packages/kartik-v/yii2-helpers)
 [![Monthly Downloads](https://poser.pugx.org/kartik-v/yii2-helpers/d/monthly)](https://packagist.org/packages/kartik-v/yii2-helpers)
@@ -12,7 +13,7 @@ This extension is a collection of useful helper functions for Yii Framework 2.0.
 ### Html Class
 [```VIEW DEMO```](http://demos.krajee.com/helper-functions/html)  
 
-This class extends the [Yii Html Helper](https://github.com/yiisoft/yii2/blob/master/framework/helpers/Html.php) to incorporate additional HTML markup functionality and features available in [Bootstrap 3.0](http://getbootstrap.com/). The helper functions available in this class are:
+This class extends the [Yii Html Helper](https://github.com/yiisoft/yii2/blob/master/framework/helpers/Html.php) to incorporate additional HTML markup functionality and features available in [Bootstrap 3.x / 4.x](http://getbootstrap.com/). The helper functions available in this class are:
 - Icon
 - Label
 - Badge
@@ -92,4 +93,4 @@ echo Enum::properize('Chris');
 
 ## License
 
-**yii2-helpers** is released under the BSD 3-Clause License. See the bundled `LICENSE.md` for details.
+**yii2-helpers** is released under the BSD-3-Clause License. See the bundled `LICENSE.md` for details.
diff --git a/composer.json b/composer.json
index 3f502bb..b2e92ad 100644
--- a/composer.json
+++ b/composer.json
@@ -12,13 +12,13 @@
             "homepage": "http://www.krajee.com/"
         }
     ],
-    "minimum-stability": "stable",
     "require": {
+        "kartik-v/yii2-krajee-base": ">=1.9",
         "yiisoft/yii2-bootstrap": "*"
     },
     "autoload": {
         "psr-4": {
-            "kartik\\helpers\\": ""
+            "kartik\\helpers\\": "src"
         }
     },
     "extra": {
diff --git a/Enum.php b/src/Enum.php
similarity index 92%
rename from Enum.php
rename to src/Enum.php
index ee18330..6a9371e 100644
--- a/Enum.php
+++ b/src/Enum.php
@@ -1,9 +1,9 @@
 <?php
 
 /**
- * @copyright Copyright &copy; Kartik Visweswaran, Krajee.com, 2013 - 2016
+ * @copyright Copyright &copy; Kartik Visweswaran, Krajee.com, 2013 - 2018
  * @package yii2-helpers
- * @version 1.3.6
+ * @version 1.3.7
  */
 
 namespace kartik\helpers;
@@ -34,7 +34,7 @@ class Enum extends Inflector
         'day' => 86400,
         'hour' => 3600,
         'minute' => 60,
-        'second' => 1
+        'second' => 1,
     ];
 
     /**
@@ -69,7 +69,7 @@ public static function isEmpty($var)
      * ~~~
      *
      * @param string $needle the value to search
-     * @param array  $haystack the array to scan
+     * @param array $haystack the array to scan
      *
      * @return boolean
      */
@@ -109,7 +109,7 @@ public static function properize($string)
      * ~~~
      *
      * @param string $fromTime start date time
-     * @param boolean   $human if true returns an approximate human friendly output. If set to `false`, will attempt an
+     * @param boolean $human if true returns an approximate human friendly output. If set to `false`, will attempt an
      *     exact conversion of time intervals.
      * @param string $toTime end date time (defaults to current system time)
      * @param string $append the string to append for the converted elapsed time. Defaults to ' ago'.
@@ -144,7 +144,7 @@ public static function initI18N()
         Yii::$app->i18n->translations['kvenum*'] = [
             'class' => 'yii\i18n\PhpMessageSource',
             'basePath' => "@kvenum/messages",
-            'forceTranslation' => true
+            'forceTranslation' => true,
         ];
     }
 
@@ -160,7 +160,7 @@ public static function initI18N()
      * ~~~
      *
      * @param integer $interval time interval in seconds
-     * @param string  $append the string to append for the converted elapsed time. Defaults to `' ago'`.
+     * @param string $append the string to append for the converted elapsed time. Defaults to `' ago'`.
      * @param boolean $human if true returns an approximate human friendly output. If set to `false`, will attempt an
      * exact conversion of time intervals.
      *
@@ -179,7 +179,7 @@ public static function timeInterval($interval, $append = null, $human = true)
             if ($interval <= 0) {
                 $elapsed = Yii::t('kvenum', 'a moment');
             } elseif ($interval < 60) {
-                $elapsed = Yii::t('kvenum', '{n, plural, one{one second} other{# seconds}}', [ 'n' => $interval]);
+                $elapsed = Yii::t('kvenum', '{n, plural, one{one second} other{# seconds}}', ['n' => $interval]);
             } elseif ($interval >= 60 && $interval < $intervals['hour']) {
                 $interval = floor($interval / $intervals['minute']);
                 $elapsed = Yii::t('kvenum', '{n, plural, one{one minute} other{# minutes}}', ['n' => $interval]);
@@ -211,7 +211,7 @@ public static function timeInterval($interval, $append = null, $human = true)
      * Example Output:
      *    1 year 5 months 3 days ago
      *
-     * @param integer   $time elapsed number of seconds
+     * @param integer $time elapsed number of seconds
      * @param array $intervals configuration of time intervals in seconds
      *
      * @return string
@@ -240,7 +240,7 @@ protected static function time2String($time, $intervals)
      * echo Enum::formatBytes(17328347842.25, 3); // returns: 16.138 GB
      * ~~~
      *
-     * @param double  $bytes number of bytes
+     * @param double $bytes number of bytes
      * @param integer $precision the number of decimal places to round off
      *
      * @return string
@@ -359,7 +359,7 @@ public static function ones()
             ' ' . Yii::t('kvenum', 'sixteen'),
             ' ' . Yii::t('kvenum', 'seventeen'),
             ' ' . Yii::t('kvenum', 'eighteen'),
-            ' ' . Yii::t('kvenum', 'nineteen')
+            ' ' . Yii::t('kvenum', 'nineteen'),
         ];
     }
 
@@ -391,7 +391,7 @@ public static function tens()
             ' ' . Yii::t('kvenum', 'sixty'),
             ' ' . Yii::t('kvenum', 'seventy'),
             ' ' . Yii::t('kvenum', 'eighty'),
-            ' ' . Yii::t('kvenum', 'ninety')
+            ' ' . Yii::t('kvenum', 'ninety'),
         ];
     }
 
@@ -451,7 +451,7 @@ public static function days()
             Yii::t('kvenum', 'Wednesday'),
             Yii::t('kvenum', 'Thursday'),
             Yii::t('kvenum', 'Friday'),
-            Yii::t('kvenum', 'Saturday')
+            Yii::t('kvenum', 'Saturday'),
         ];
     }
 
@@ -527,8 +527,8 @@ public static function yearList($from, $to = null, $keys = false, $desc = true)
      * Generate a month or day array list for Gregorian calendar
      *
      * @param string $unit whether 'day' or 'month'
-     * @param boolean   $abbr whether to return abbreviated day or month
-     * @param integer    $start the first day or month to set. Defaults to `1`.
+     * @param boolean $abbr whether to return abbreviated day or month
+     * @param integer $start the first day or month to set. Defaults to `1`.
      * @param string $case whether 'upper', lower', or null. If null, then the initcap case will be used.
      *
      * @return array list of days or months
@@ -572,8 +572,8 @@ protected static function genCalList($unit = 'day', $abbr = false, $start = 1, $
      * print_r(Enum::monthList(true, 4, 'upper'));
      * ~~~
      *
-     * @param boolean   $abbr whether to return abbreviated month
-     * @param integer    $start the first month to set. Defaults to `1` for `January`.
+     * @param boolean $abbr whether to return abbreviated month
+     * @param integer $start the first month to set. Defaults to `1` for `January`.
      * @param string $case whether 'upper', lower', or null. If null, then the initcap case will be used.
      *
      * @return array list of months
@@ -586,8 +586,8 @@ public static function monthList($abbr = false, $start = 1, $case = null)
     /**
      * Generate a day array list for Gregorian calendar
      *
-     * @param boolean   $abbr whether to return abbreviated day
-     * @param integer    $start the first day to set. Defaults to `1` for `Sunday`.
+     * @param boolean $abbr whether to return abbreviated day
+     * @param integer $start the first day to set. Defaults to `1` for `Sunday`.
      * @param string $case whether 'upper', lower', or null. If null, then the initcap case will be used.
      *
      * @return array list of days
@@ -607,9 +607,9 @@ public static function dayList($abbr = false, $start = 1, $case = null)
      * print_r(Enum::dateList(1, 10));
      * ~~~
      *
-     * @param integer  $from the start day, defaults to 1
-     * @param integer  $to the end day, defaults to 31
-     * @param integer  $interval the date interval, defaults to 1.
+     * @param integer $from the start day, defaults to 1
+     * @param integer $to the end day, defaults to 31
+     * @param integer $interval the date interval, defaults to 1.
      * @param boolean $intervalFromZero whether to start incrementing intervals from zero if $from = 1.
      * @param boolean $showLast whether to show the last date (set in $to) even if it does not match interval.
      *
@@ -651,11 +651,11 @@ public static function dateList($from = 1, $to = 31, $interval = 1, $intervalFro
      * echo implode(', ', Enum::timeList('sec', 5));
      * ~~~
      *
-     * @param string  $unit the time unit ('hour', 'min', 'sec', 'ms')
+     * @param string $unit the time unit ('hour', 'min', 'sec', 'ms')
      * @param integer $interval the time interval.
      * @param integer $from the time from (defaults to 23 for hour
      * @param integer $to the time to (defaults to 1).
-     * @param boolean    $padZero whether to pad zeros to the left of each time unit value.
+     * @param boolean $padZero whether to pad zeros to the left of each time unit value.
      *
      * @return array
      * @throws InvalidConfigException if $unit passed is invalid
@@ -734,7 +734,7 @@ public static function boolList($false = null, $true = null)
      * echo Enum::array2table($data);
      * ~~~
      *
-     * @param array  $array the associative array to be converted
+     * @param array $array the associative array to be converted
      * @param boolean $transpose whether to show keys as rows instead of columns. This parameter should be used only
      * for a single dimensional associative array. If used for a multidimensional array, the sub array will be imploded
      * as text.
@@ -787,7 +787,7 @@ public static function array2table(
         }
 
         if (!isset($array[0]) || !is_array($array[0])) {
-            $array = array($array);
+            $array = [$array];
         }
         // Take the keys from the first row as the headings
         foreach (array_keys($array[0]) as $heading) {
@@ -894,32 +894,14 @@ public static function getType($var)
      */
     public static function userIP($filterLocal = true)
     {
-        $ipSources = [
-            'HTTP_CLIENT_IP',
-            'HTTP_X_FORWARDED_FOR',
-            'HTTP_X_FORWARDED',
-            'HTTP_X_CLUSTER_CLIENT_IP',
-            'HTTP_FORWARDED_FOR',
-            'HTTP_FORWARDED',
-            'REMOTE_ADDR'
-        ];
-        foreach ($ipSources as $key) {
-            if (array_key_exists($key, $_SERVER) === true) {
-                foreach (array_map('trim', explode(',', $_SERVER[$key])) as $ip) {
-                    if ($filterLocal) {
-                        $checkFilter = filter_var(
-                            $ip,
-                            FILTER_VALIDATE_IP,
-                            FILTER_FLAG_NO_PRIV_RANGE | FILTER_FLAG_NO_RES_RANGE
-                        );
-                        if ($checkFilter !== false) {
-                            return $ip;
-                        }
-                    } else {
-                        return $ip;
-                    }
-                }
+        $ip = Yii::$app->request->getUserIP();
+        if ($filterLocal) {
+            $checkFilter = filter_var($ip, FILTER_VALIDATE_IP, FILTER_FLAG_NO_PRIV_RANGE | FILTER_FLAG_NO_RES_RANGE);
+            if ($checkFilter !== false) {
+                return $ip;
             }
+        } else {
+            return $ip;
         }
         return 'Unknown';
     }
@@ -934,8 +916,8 @@ public static function userIP($filterLocal = true)
      * ~~~
      *
      * @param boolean $common show common browsers only
-     * @param array   $browsers the list of browsers
-     * @param string  $agent user agent
+     * @param array $browsers the list of browsers
+     * @param string $agent user agent
      *
      * @return array the browser information
      */
@@ -988,7 +970,7 @@ public static function getBrowser($common = false, $browsers = [], $agent = null
             'code' => 'other',
             'name' => Yii::t('kvenum', 'Other'),
             'version' => "?",
-            'platform' => Yii::t('kvenum', 'Unknown')
+            'platform' => Yii::t('kvenum', 'Unknown'),
         ];
         $ios = Yii::t('kvenum', 'iOS');
         $unix = Yii::t('kvenum', 'Unix');
diff --git a/Html.php b/src/Html.php
similarity index 87%
rename from Html.php
rename to src/Html.php
index 29cfe86..238f3ca 100644
--- a/Html.php
+++ b/src/Html.php
@@ -1,18 +1,21 @@
 <?php
 
 /**
- * @copyright Copyright &copy; Kartik Visweswaran, Krajee.com, 2013 - 2016
+ * @copyright Copyright &copy; Kartik Visweswaran, Krajee.com, 2013 - 2018
  * @package yii2-helpers
- * @version 1.3.6
+ * @version 1.3.7
  */
 
 namespace kartik\helpers;
 
 use Closure;
+use kartik\base\Widget;
 use Yii;
+use yii\base\InvalidConfigException;
 use yii\base\Model;
 use yii\helpers\ArrayHelper;
 use yii\helpers\Html as YiiHtml;
+use yii\widgets\ActiveField;
 
 /**
  * Html provides a set of static methods for generating commonly used HTML tags and extends [[YiiHtml]]
@@ -114,6 +117,21 @@ class Html extends YiiHtml
      */
     const TYPE_SUCCESS = 'success';
 
+    /**
+     * Check if HTML options has specified CSS class
+     * @param array $options the HTML options
+     * @param string $cssClass the css class to test
+     * @return bool
+     */
+    protected static function hasCssClass($options, $cssClass)
+    {
+        if (!isset($options['class'])) {
+            return false;
+        }
+        $classes = preg_split('/\s+/', $options['class'], -1, PREG_SPLIT_NO_EMPTY);
+        return in_array($cssClass, $classes);
+    }
+
     /**
      * Generates a bootstrap icon markup.
      *
@@ -158,18 +176,24 @@ public static function icon($icon, $options = [], $prefix = 'glyphicon glyphicon
      * @param string $type the bootstrap label type. Defaults to 'default'. Should be one of the bootstrap contextual
      * colors: 'default, 'primary', 'success', 'info', 'danger', 'warning'.
      * @param array $options HTML attributes / options for the label container
-     * @param string $prefix the CSS class prefix. Defaults to 'label label-'.
      * @param string $tag the label container tag. Defaults to 'span'.
+     * @param string $prefix the CSS class prefix. Defaults to null and will be auto parsed based on BS Version.
      *
      * @return string
+     * @throws InvalidConfigException
      */
-    public static function bsLabel($content, $type = 'default', $options = [], $prefix = 'label label-', $tag = 'span')
+    public static function bsLabel($content, $type = 'default', $options = [], $tag = 'span', $prefix = null)
     {
+        $widget = new Widget();
         if (Enum::isEmpty($type)) {
             $type = self::TYPE_DEFAULT;
         }
-        $class = isset($options['class']) ? ' ' . $options['class'] : '';
-        $options['class'] = $prefix . $type . $class;
+        if (isset($prefix)) {
+            static::addCssClass($options, $prefix . $type);
+        } else {
+            $widget->addCssClass($options, Widget::BS_LABEL);
+            $widget->addCssClass($options, "label-{$type}");
+        }
         return static::tag($tag, $content, $options);
     }
 
@@ -190,10 +214,12 @@ public static function bsLabel($content, $type = 'default', $options = [], $pref
      * @param string $tag the label container tag. Defaults to 'span'.
      *
      * @return string
+     * @throws InvalidConfigException
      */
     public static function badge($content, $options = [], $tag = 'span')
     {
-        static::addCssClass($options, 'badge');
+        $widget = new Widget();
+        $widget->addCssClass($options, Widget::BS_BADGE);
         return static::tag($tag, $content, $options);
     }
 
@@ -256,20 +282,29 @@ public static function badge($content, $options = [], $tag = 'span')
      * - `badge`: _string_, any badge content to be displayed for this list item (optional)
      * - `badgeOptions`: _array_, the HTML attributes / options for badge container (optional).
      * - `active`: _boolean_, to highlight the item as active (applicable only if $url is passed). Defaults to `false`.
-     * - `options`: _array_, HTML attributes / options for the list group item container (optional).
-     * @param array $options HTML attributes / options for the list group container
-     * @param string $tag the list group container tag. Defaults to 'div'.
-     * @param string $itemTag the list item container tag. Defaults to 'div'.
+     * - `options`: _array_, HTML attributes / options for the list group item container (optional). The following
+     *    special option is recognized:
+     *    - `tag`: _string_, the list group container tag. Defaults to 'div'.
+     * @param array $options HTML attributes for the list group container. The following special option is recognized:
+     * - `tag`: _string_, the list group container tag. Defaults to 'div'.
+     * @param array $itemOptions HTML attributes for the list group item options (will be overridden by `options` at
+     *   the item level). The following special option is recognized:
+     * - `tag`: _string_, the list group container tag. Defaults to 'div'.
+     * @param array $badgeOptions HTML attributes for the list group item badge options (will be overridden by
+     * `badgeOptions` at the item level)
      *
      * @return string
+     * @return string
+     * @throws  InvalidConfigException
      */
-    public static function listGroup($items = [], $options = [], $tag = 'div', $itemTag = 'div')
+    public static function listGroup($items = [], $options = [], $itemOptions = [], $badgeOptions = [])
     {
         static::addCssClass($options, 'list-group');
         $content = '';
         foreach ($items as $item) {
-            $content .= static::getListGroupItem($item, $itemTag) . "\n";
+            $content .= static::getListGroupItem($item, $itemOptions, $badgeOptions) . "\n";
         }
+        $tag = ArrayHelper::remove($options, 'tag', 'div');
         return static::tag($tag, $content, $options);
     }
 
@@ -402,19 +437,26 @@ public static function jumbotron($content = [], $fullWidth = false, $options = [
      * @param string $prefix the CSS prefix for panel type. Defaults to `panel panel-`.
      *
      * @return string
+     * @throws InvalidConfigException
      */
-    public static function panel($content = [], $type = 'default', $options = [], $prefix = 'panel panel-')
+    public static function panel($content = [], $type = 'default', $options = [], $prefix = null)
     {
         if (!is_array($content)) {
             return '';
         } else {
-            static::addCssClass($options, $prefix . $type);
+            $widget = new Widget();
+            if (isset($prefix)) {
+                static::addCssClass($options, $prefix . $type);
+            } else {
+                $widget->addCssClass($options, Widget::BS_PANEL);
+                $widget->addCssClass($options, "panel-{$type}");
+            }
             $panel = static::getPanelContent($content, 'preHeading') .
-                static::getPanelTitle($content, 'heading') .
+                static::getPanelTitle($content, 'heading', $widget) .
                 static::getPanelContent($content, 'preBody') .
                 static::getPanelContent($content, 'body') .
                 static::getPanelContent($content, 'postBody') .
-                static::getPanelTitle($content, 'footer') .
+                static::getPanelTitle($content, 'footer', $widget) .
                 static::getPanelContent($content, 'postFooter');
             return static::tag('div', $panel, $options);
         }
@@ -458,19 +500,25 @@ public static function pageHeader($title, $subTitle = '', $options = [])
      *
      * @param string $content the content
      * @param string $size the well size. Should be one of the bootstrap size modifiers:
-     * - [[SIZE_TINY]] or `xs`
      * - [[SIZE_SMALL]] or `sm`
-     * - [[SIZE_MEDIUM]] or `md`
      * - [[SIZE_LARGE]] or `lg`
      * @param array $options HTML attributes / options for the well container.
      *
      * @return string
+     * @throws InvalidConfigException
      */
     public static function well($content, $size = '', $options = [])
     {
-        static::addCssClass($options, 'well');
+        $widget = new Widget();
         if (!Enum::isEmpty($size)) {
-            static::addCssClass($options, 'well-' . $size);
+            if ($size === self::SIZE_SMALL) {
+                $widget->addCssClass($options, Widget::BS_WELL_SM);
+            }
+            if ($size === self::SIZE_LARGE) {
+                $widget->addCssClass($options, Widget::BS_WELL_LG);
+            }
+        } else {
+            $widget->addCssClass($options, Widget::BS_WELL);
         }
         return static::tag('div', $content, $options);
     }
@@ -714,16 +762,42 @@ public static function abbr($content, $title, $initialism = false, $options = []
      * '{source}' to embed the cite source
      * @param string $citeTitle the cite source title (optional)
      * @param string $citeSource the cite source (optional)
-     * @param array $options html options for the blockquote
-     *
+     * @param array $options HTML attributes for the blockquote main container
+     * @param array $contentOptions HTML attributes for the blockquote content container. For Bootstrap 4.x, the CSS
+     * class `mb-0` will be set if the class attribute is not set. The following option is specially recognized:
+     *  - `tag`: _string_, the tag to render the content. Defaults to `p`.
+     * @param array $footerOptions HTML attributes for the blockquote footer container. Applicable only for Bootstrap
+     * 4.x releases.
      * @return string
+     * @throws InvalidConfigException
      */
-    public static function blockquote($content, $citeContent = '', $citeTitle = '', $citeSource = '', $options = [])
-    {
-        $content = static::tag('p', $content);
+    public static function blockquote(
+        $content,
+        $citeContent = '',
+        $citeTitle = '',
+        $citeSource = '',
+        $options = [],
+        $contentOptions = [],
+        $footerOptions = []
+    ) {
+        $widget = new Widget();
+        $isBs4 = $widget->isBs4();
+        if ($isBs4) {
+            static::addCssClass($options, 'blockquote');
+            if (!isset($contentOptions['class'])) {
+                $contentOptions['class'] = 'mb-0';
+            }
+        }
+        $tag = ArrayHelper::remove($contentOptions, 'tag', 'p');
+        $content = static::tag($tag, $content, $contentOptions);
         if (!Enum::isEmpty($citeContent)) {
             $source = static::tag('cite', $citeSource, ['title' => $citeTitle]);
-            $content .= "\n<small>" . str_replace('{source}', $source, $citeContent) . "</small>";
+            if ($isBs4) {
+                static::addCssClass($footerOptions, 'blockquote-footer');
+                $content .= Html::tag('div', str_replace('{source}', $source, $citeContent), $footerOptions);
+            } else {
+                $content .= "\n<small>" . str_replace('{source}', $source, $citeContent) . "</small>";
+            }
         }
         return static::tag('blockquote', $content, $options);
     }
@@ -842,20 +916,21 @@ public static function address(
     public static function getButtonGroup($type, $name, $selection = null, $items = [], $options = [])
     {
         $class = $type . 'List';
-        static::addCssClass($options, 'btn-group');
+        static::addCssClass($options, ['btn-group', 'btn-group-toggle']);
         $options['data-toggle'] = 'buttons';
         $options['inline'] = true;
+        $widget = new Widget();
         if (!isset($options['itemOptions']['labelOptions']['class'])) {
-            $options['itemOptions']['labelOptions']['class'] = 'btn btn-default';
+            $options['itemOptions']['labelOptions']['class'] = 'btn ' . $widget->getDefaultBtnCss();
         }
         if (!isset($options['item']) || !$options['item'] instanceof Closure) {
-            /** @noinspection PhpUnusedParameterInspection */
             /**
              * @param string $index
              * @param string $label
              * @param string $name
              * @param boolean $checked
              * @param string $value
+             * @return ActiveField
              */
             $options['item'] = function ($index, $label, $name, $checked, $value) use ($type, $options) {
                 $opts = isset($options['itemOptions']) ? $options['itemOptions'] : [];
@@ -1083,16 +1158,28 @@ public static function activeRadioButtonGroup($model, $attribute, $items, $optio
      * - `badgeOptions`: _array_, the HTML attributes / options for badge container (optional).
      * - `active`: _boolean_, to highlight the item as active (applicable only if $url is passed). Defaults to `false`.
      * - `options`: _array_, HTML attributes / options for the list group item container (optional).
-     * @param string $tag the list item container tag (applied if it is not a link)
+     * @param array $itemOptions HTML attributes for the list group item options (will be overridden by `options` at
+     *   the item level). The following special option is recognized:
+     * - `tag`: _string_, the list group container tag. Defaults to 'div'.
+     * @param array $badgeOptions HTML attributes for the list group item badge options (will be overridden by
+     * `badgeOptions` at the item level)
      *
      * @return string
+     * @throws InvalidConfigException
      */
-    protected static function getListGroupItem($item, $tag)
+    protected static function getListGroupItem($item, $itemOptions = [], $badgeOptions = [])
     {
-        static::addCssClass($item['options'], 'list-group-item');
+        $widget = new Widget();
+        $isBs4 = $widget->isBs4();
         $heading = $body = $badge = $content = $url = $active = '';
-        $options = $headingOptions = $bodyOptions = $badgeOptions = [];
-        extract($item);
+        $itemOptions += ArrayHelper::getValue($item, 'options', []);
+        $badgeOptions += ArrayHelper::getValue($item, 'badgeOptions', []);
+        $url = ArrayHelper::getValue($item, 'url');
+        $badge = ArrayHelper::getValue($item, 'badge', '');
+        $content = ArrayHelper::getValue($item, 'content', '');
+        $active = ArrayHelper::getValue($item, 'active', false);
+        static::addCssClass($itemOptions, 'list-group-item');
+        $hasUrl = !Enum::isEmpty($url);
         if (is_array($content)) {
             extract($content);
             if (!Enum::isEmpty($heading)) {
@@ -1106,15 +1193,23 @@ protected static function getListGroupItem($item, $tag)
             $content = $heading . "\n" . $body;
         }
         if (!Enum::isEmpty($badge)) {
-            $content = static::badge($badge, $badgeOptions) . $content;
+            if ($isBs4 && empty($badgeOptions['class'])) {
+                $badgeOptions['class'] = 'badge-secondary';
+            }
+            $badge = static::badge($badge, $badgeOptions);
+            $content = $isBs4 ? $content . $badge : $badge . $content;
+        }
+        $tag = ArrayHelper::remove($itemOptions, 'tag', 'div');
+        if ($active) {
+            static::addCssClass($itemOptions, 'active');
         }
-        if (!Enum::isEmpty($url)) {
-            if ($active) {
-                static::addCssClass($options, 'active');
+        if ($hasUrl) {
+            if ($isBs4) {
+                static::addCssClass($itemOptions, 'list-group-item-action');
             }
-            return static::a($content, $url, $options);
+            return static::a($content, $url, $itemOptions);
         } else {
-            return static::tag($tag, $content, $options);
+            return static::tag($tag, $content, $itemOptions);
         }
     }
 
@@ -1137,17 +1232,22 @@ protected static function getPanelContent($content, $type)
      *
      * @param array $content the panel content settings.
      * @param string $type whether `heading` or `footer`
+     * @param Widget $widget the widget object
      *
      * @return string
+     * @throws InvalidConfigException
      */
-    protected static function getPanelTitle($content, $type)
+    protected static function getPanelTitle($content, $type, $widget)
     {
         $title = ArrayHelper::getValue($content, $type, '');
         if (!Enum::isEmpty($title)) {
             if (ArrayHelper::getValue($content, "{$type}Title", true) === true) {
-                $title = static::tag("h3", $title, ["class" => "panel-title"]);
+                $isBs4 = $widget->isBs4();
+                $tag = $isBs4 ? 'h5' : 'h3';
+                $css = $isBs4 ? 'm-0' : 'panel-title';
+                $title = static::tag($tag, $title, ['class' => $css]);
             }
-            return static::tag("div", $title, ["class" => "panel-{$type}"]) . "\n";
+            return static::tag("div", $title, ["class" => $widget->getCssClass("panel-{$type}")]) . "\n";
         } else {
             return '';
         }
diff --git a/messages/af/kvenum.php b/src/messages/af/kvenum.php
similarity index 100%
rename from messages/af/kvenum.php
rename to src/messages/af/kvenum.php
diff --git a/messages/ar/kvenum.php b/src/messages/ar/kvenum.php
similarity index 100%
rename from messages/ar/kvenum.php
rename to src/messages/ar/kvenum.php
diff --git a/messages/az/kvenum.php b/src/messages/az/kvenum.php
similarity index 100%
rename from messages/az/kvenum.php
rename to src/messages/az/kvenum.php
diff --git a/messages/bg/kvenum.php b/src/messages/bg/kvenum.php
similarity index 100%
rename from messages/bg/kvenum.php
rename to src/messages/bg/kvenum.php
diff --git a/messages/bs/kvenum.php b/src/messages/bs/kvenum.php
similarity index 100%
rename from messages/bs/kvenum.php
rename to src/messages/bs/kvenum.php
diff --git a/messages/ca/kvenum.php b/src/messages/ca/kvenum.php
similarity index 100%
rename from messages/ca/kvenum.php
rename to src/messages/ca/kvenum.php
diff --git a/messages/config.php b/src/messages/config.php
similarity index 100%
rename from messages/config.php
rename to src/messages/config.php
diff --git a/messages/cs/kvenum.php b/src/messages/cs/kvenum.php
similarity index 100%
rename from messages/cs/kvenum.php
rename to src/messages/cs/kvenum.php
diff --git a/messages/da/kvenum.php b/src/messages/da/kvenum.php
similarity index 100%
rename from messages/da/kvenum.php
rename to src/messages/da/kvenum.php
diff --git a/messages/de/kvenum.php b/src/messages/de/kvenum.php
similarity index 100%
rename from messages/de/kvenum.php
rename to src/messages/de/kvenum.php
diff --git a/messages/el/kvenum.php b/src/messages/el/kvenum.php
similarity index 100%
rename from messages/el/kvenum.php
rename to src/messages/el/kvenum.php
diff --git a/messages/en/kvenum.php b/src/messages/en/kvenum.php
similarity index 100%
rename from messages/en/kvenum.php
rename to src/messages/en/kvenum.php
diff --git a/messages/eo/kvenum.php b/src/messages/eo/kvenum.php
similarity index 100%
rename from messages/eo/kvenum.php
rename to src/messages/eo/kvenum.php
diff --git a/messages/es/kvenum.php b/src/messages/es/kvenum.php
similarity index 100%
rename from messages/es/kvenum.php
rename to src/messages/es/kvenum.php
diff --git a/messages/et/kvenum.php b/src/messages/et/kvenum.php
similarity index 100%
rename from messages/et/kvenum.php
rename to src/messages/et/kvenum.php
diff --git a/messages/eu/kvenum.php b/src/messages/eu/kvenum.php
similarity index 100%
rename from messages/eu/kvenum.php
rename to src/messages/eu/kvenum.php
diff --git a/messages/fa/kvenum.php b/src/messages/fa/kvenum.php
similarity index 100%
rename from messages/fa/kvenum.php
rename to src/messages/fa/kvenum.php
diff --git a/messages/fi/kvenum.php b/src/messages/fi/kvenum.php
similarity index 100%
rename from messages/fi/kvenum.php
rename to src/messages/fi/kvenum.php
diff --git a/messages/fo/kvenum.php b/src/messages/fo/kvenum.php
similarity index 100%
rename from messages/fo/kvenum.php
rename to src/messages/fo/kvenum.php
diff --git a/messages/fr/kvenum.php b/src/messages/fr/kvenum.php
similarity index 100%
rename from messages/fr/kvenum.php
rename to src/messages/fr/kvenum.php
diff --git a/messages/gl/kvenum.php b/src/messages/gl/kvenum.php
similarity index 100%
rename from messages/gl/kvenum.php
rename to src/messages/gl/kvenum.php
diff --git a/messages/gu/kvenum.php b/src/messages/gu/kvenum.php
similarity index 100%
rename from messages/gu/kvenum.php
rename to src/messages/gu/kvenum.php
diff --git a/messages/he/kvenum.php b/src/messages/he/kvenum.php
similarity index 100%
rename from messages/he/kvenum.php
rename to src/messages/he/kvenum.php
diff --git a/messages/hi/kvenum.php b/src/messages/hi/kvenum.php
similarity index 100%
rename from messages/hi/kvenum.php
rename to src/messages/hi/kvenum.php
diff --git a/messages/hr/kvenum.php b/src/messages/hr/kvenum.php
similarity index 100%
rename from messages/hr/kvenum.php
rename to src/messages/hr/kvenum.php
diff --git a/messages/hu/kvenum.php b/src/messages/hu/kvenum.php
similarity index 100%
rename from messages/hu/kvenum.php
rename to src/messages/hu/kvenum.php
diff --git a/messages/hy/kvenum.php b/src/messages/hy/kvenum.php
similarity index 100%
rename from messages/hy/kvenum.php
rename to src/messages/hy/kvenum.php
diff --git a/messages/id/kvenum.php b/src/messages/id/kvenum.php
similarity index 100%
rename from messages/id/kvenum.php
rename to src/messages/id/kvenum.php
diff --git a/messages/is/kvenum.php b/src/messages/is/kvenum.php
similarity index 100%
rename from messages/is/kvenum.php
rename to src/messages/is/kvenum.php
diff --git a/messages/it/kvenum.php b/src/messages/it/kvenum.php
similarity index 100%
rename from messages/it/kvenum.php
rename to src/messages/it/kvenum.php
diff --git a/messages/ja/kvenum.php b/src/messages/ja/kvenum.php
similarity index 100%
rename from messages/ja/kvenum.php
rename to src/messages/ja/kvenum.php
diff --git a/messages/ka/kvenum.php b/src/messages/ka/kvenum.php
similarity index 100%
rename from messages/ka/kvenum.php
rename to src/messages/ka/kvenum.php
diff --git a/messages/kk/kvenum.php b/src/messages/kk/kvenum.php
similarity index 100%
rename from messages/kk/kvenum.php
rename to src/messages/kk/kvenum.php
diff --git a/messages/kn/kvenum.php b/src/messages/kn/kvenum.php
similarity index 100%
rename from messages/kn/kvenum.php
rename to src/messages/kn/kvenum.php
diff --git a/messages/ko/kvenum.php b/src/messages/ko/kvenum.php
similarity index 100%
rename from messages/ko/kvenum.php
rename to src/messages/ko/kvenum.php
diff --git a/messages/ky/kvenum.php b/src/messages/ky/kvenum.php
similarity index 100%
rename from messages/ky/kvenum.php
rename to src/messages/ky/kvenum.php
diff --git a/messages/lt/kvenum.php b/src/messages/lt/kvenum.php
similarity index 100%
rename from messages/lt/kvenum.php
rename to src/messages/lt/kvenum.php
diff --git a/messages/lv/kvenum.php b/src/messages/lv/kvenum.php
similarity index 100%
rename from messages/lv/kvenum.php
rename to src/messages/lv/kvenum.php
diff --git a/messages/mi/kvenum.php b/src/messages/mi/kvenum.php
similarity index 100%
rename from messages/mi/kvenum.php
rename to src/messages/mi/kvenum.php
diff --git a/messages/mk/kvenum.php b/src/messages/mk/kvenum.php
similarity index 100%
rename from messages/mk/kvenum.php
rename to src/messages/mk/kvenum.php
diff --git a/messages/mn/kvenum.php b/src/messages/mn/kvenum.php
similarity index 100%
rename from messages/mn/kvenum.php
rename to src/messages/mn/kvenum.php
diff --git a/messages/mr/kvenum.php b/src/messages/mr/kvenum.php
similarity index 100%
rename from messages/mr/kvenum.php
rename to src/messages/mr/kvenum.php
diff --git a/messages/ms/kvenum.php b/src/messages/ms/kvenum.php
similarity index 100%
rename from messages/ms/kvenum.php
rename to src/messages/ms/kvenum.php
diff --git a/messages/mt/kvenum.php b/src/messages/mt/kvenum.php
similarity index 100%
rename from messages/mt/kvenum.php
rename to src/messages/mt/kvenum.php
diff --git a/messages/nb/kvenum.php b/src/messages/nb/kvenum.php
similarity index 100%
rename from messages/nb/kvenum.php
rename to src/messages/nb/kvenum.php
diff --git a/messages/nl/kvenum.php b/src/messages/nl/kvenum.php
similarity index 100%
rename from messages/nl/kvenum.php
rename to src/messages/nl/kvenum.php
diff --git a/messages/nn/kvenum.php b/src/messages/nn/kvenum.php
similarity index 100%
rename from messages/nn/kvenum.php
rename to src/messages/nn/kvenum.php
diff --git a/messages/pl/kvenum.php b/src/messages/pl/kvenum.php
similarity index 100%
rename from messages/pl/kvenum.php
rename to src/messages/pl/kvenum.php
diff --git a/messages/pt-BR/kvenum.php b/src/messages/pt-BR/kvenum.php
similarity index 100%
rename from messages/pt-BR/kvenum.php
rename to src/messages/pt-BR/kvenum.php
diff --git a/messages/pt/kvenum.php b/src/messages/pt/kvenum.php
similarity index 100%
rename from messages/pt/kvenum.php
rename to src/messages/pt/kvenum.php
diff --git a/messages/qu/kvenum.php b/src/messages/qu/kvenum.php
similarity index 100%
rename from messages/qu/kvenum.php
rename to src/messages/qu/kvenum.php
diff --git a/messages/ro/kvenum.php b/src/messages/ro/kvenum.php
similarity index 100%
rename from messages/ro/kvenum.php
rename to src/messages/ro/kvenum.php
diff --git a/messages/ru/kvenum.php b/src/messages/ru/kvenum.php
similarity index 100%
rename from messages/ru/kvenum.php
rename to src/messages/ru/kvenum.php
diff --git a/messages/sa/kvenum.php b/src/messages/sa/kvenum.php
similarity index 100%
rename from messages/sa/kvenum.php
rename to src/messages/sa/kvenum.php
diff --git a/messages/se/kvenum.php b/src/messages/se/kvenum.php
similarity index 100%
rename from messages/se/kvenum.php
rename to src/messages/se/kvenum.php
diff --git a/messages/sk/kvenum.php b/src/messages/sk/kvenum.php
similarity index 100%
rename from messages/sk/kvenum.php
rename to src/messages/sk/kvenum.php
diff --git a/messages/sl/kvenum.php b/src/messages/sl/kvenum.php
similarity index 100%
rename from messages/sl/kvenum.php
rename to src/messages/sl/kvenum.php
diff --git a/messages/sq/kvenum.php b/src/messages/sq/kvenum.php
similarity index 100%
rename from messages/sq/kvenum.php
rename to src/messages/sq/kvenum.php
diff --git a/messages/sr-Latn/kvenum.php b/src/messages/sr-Latn/kvenum.php
similarity index 100%
rename from messages/sr-Latn/kvenum.php
rename to src/messages/sr-Latn/kvenum.php
diff --git a/messages/sr/kvenum.php b/src/messages/sr/kvenum.php
similarity index 100%
rename from messages/sr/kvenum.php
rename to src/messages/sr/kvenum.php
diff --git a/messages/sv/kvenum.php b/src/messages/sv/kvenum.php
similarity index 100%
rename from messages/sv/kvenum.php
rename to src/messages/sv/kvenum.php
diff --git a/messages/sw/kvenum.php b/src/messages/sw/kvenum.php
similarity index 100%
rename from messages/sw/kvenum.php
rename to src/messages/sw/kvenum.php
diff --git a/messages/ta/kvenum.php b/src/messages/ta/kvenum.php
similarity index 100%
rename from messages/ta/kvenum.php
rename to src/messages/ta/kvenum.php
diff --git a/messages/te/kvenum.php b/src/messages/te/kvenum.php
similarity index 100%
rename from messages/te/kvenum.php
rename to src/messages/te/kvenum.php
diff --git a/messages/th/kvenum.php b/src/messages/th/kvenum.php
similarity index 100%
rename from messages/th/kvenum.php
rename to src/messages/th/kvenum.php
diff --git a/messages/tj/kvenum.php b/src/messages/tj/kvenum.php
similarity index 100%
rename from messages/tj/kvenum.php
rename to src/messages/tj/kvenum.php
diff --git a/messages/tl/kvenum.php b/src/messages/tl/kvenum.php
similarity index 100%
rename from messages/tl/kvenum.php
rename to src/messages/tl/kvenum.php
diff --git a/messages/tn/kvenum.php b/src/messages/tn/kvenum.php
similarity index 100%
rename from messages/tn/kvenum.php
rename to src/messages/tn/kvenum.php
diff --git a/messages/tr/kvenum.php b/src/messages/tr/kvenum.php
similarity index 100%
rename from messages/tr/kvenum.php
rename to src/messages/tr/kvenum.php
diff --git a/messages/ts/kvenum.php b/src/messages/ts/kvenum.php
similarity index 100%
rename from messages/ts/kvenum.php
rename to src/messages/ts/kvenum.php
diff --git a/messages/tt/kvenum.php b/src/messages/tt/kvenum.php
similarity index 100%
rename from messages/tt/kvenum.php
rename to src/messages/tt/kvenum.php
diff --git a/messages/uk/kvenum.php b/src/messages/uk/kvenum.php
similarity index 100%
rename from messages/uk/kvenum.php
rename to src/messages/uk/kvenum.php
diff --git a/messages/ur/kvenum.php b/src/messages/ur/kvenum.php
similarity index 100%
rename from messages/ur/kvenum.php
rename to src/messages/ur/kvenum.php
diff --git a/messages/vi/kvenum.php b/src/messages/vi/kvenum.php
similarity index 100%
rename from messages/vi/kvenum.php
rename to src/messages/vi/kvenum.php
diff --git a/messages/zh-CN/kvenum.php b/src/messages/zh-CN/kvenum.php
similarity index 100%
rename from messages/zh-CN/kvenum.php
rename to src/messages/zh-CN/kvenum.php
diff --git a/messages/zh-TW/kvenum.php b/src/messages/zh-TW/kvenum.php
similarity index 100%
rename from messages/zh-TW/kvenum.php
rename to src/messages/zh-TW/kvenum.php
diff --git a/messages/zu/kvenum.php b/src/messages/zu/kvenum.php
similarity index 100%
rename from messages/zu/kvenum.php
rename to src/messages/zu/kvenum.php