diff --git a/.env.example b/.env.example
index 6866e10c..a5c6e381 100644
--- a/.env.example
+++ b/.env.example
@@ -28,6 +28,7 @@ VERCEL_DEPLOY_HOOK_URL=
 
 OPENAI_TIMEOUT=
 OPENAI_API_KEY=
+OPENAI_BASE_URL=
 OPENAI_COMPLETION_MODEL=
 OPENAI_COMPLETION_TEMPERATURE=
 OPENAI_COMPLETION_MAX_TOKENS=
diff --git a/CHANGELOG.md b/CHANGELOG.md
index f6af0376..a6241041 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,11 @@
 # Changelog
 
+## 4.6.0 (2023-05-03)
+
+### New Features
+
+- Support `gpt-4` model
+
 ## 4.5.0 (2023-04-27)
 
 ### New Features
diff --git a/README.md b/README.md
index 3ca7aebe..9126a789 100644
--- a/README.md
+++ b/README.md
@@ -10,8 +10,9 @@ GPT AI Assistant is an application that is implemented using the OpenAI API and
 
 ## News
 
-- 2023-03-05: The `4.1` version now support the audio message of LINE and  `whisper-1` language model of OpenAI. :fire:
-- 2023-03-02: The `4.0` version now support `gpt-3.5-turbo` language model of OpenAI. :fire:
+- 2023-05-03: The `4.6` version now support `gpt-4` OpenAI model. :fire:
+- 2023-03-05: The `4.1` version now support the audio message of LINE and  `whisper-1` OpenAI model. :fire:
+- 2023-03-02: The `4.0` version now support `gpt-3.5-turbo` OpenAI model. :fire:
 
 ## Documentations
 
diff --git a/config/index.js b/config/index.js
index 385d8607..07a8a355 100644
--- a/config/index.js
+++ b/config/index.js
@@ -34,6 +34,7 @@ const config = Object.freeze({
   VERCEL_DEPLOY_HOOK_URL: env.VERCEL_DEPLOY_HOOK_URL || null,
   OPENAI_TIMEOUT: env.OPENAI_TIMEOUT || env.APP_API_TIMEOUT,
   OPENAI_API_KEY: env.OPENAI_API_KEY || null,
+  OPENAI_BASE_URL: env.OPENAI_BASE_URL || 'https://api.openai.com',
   OPENAI_COMPLETION_MODEL: env.OPENAI_COMPLETION_MODEL || 'gpt-3.5-turbo',
   OPENAI_COMPLETION_TEMPERATURE: Number(env.OPENAI_COMPLETION_TEMPERATURE) || 0.9,
   OPENAI_COMPLETION_MAX_TOKENS: Number(env.OPENAI_COMPLETION_MAX_TOKENS) || 160,
diff --git a/package-lock.json b/package-lock.json
index 36570507..dbec2f79 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -1,12 +1,12 @@
 {
   "name": "gpt-ai-assistant",
-  "version": "4.5.0",
+  "version": "4.6.0",
   "lockfileVersion": 2,
   "requires": true,
   "packages": {
     "": {
       "name": "gpt-ai-assistant",
-      "version": "4.5.0",
+      "version": "4.6.0",
       "dependencies": {
         "axios": "^1.2.1",
         "dotenv": "^16.0.3",
diff --git a/package.json b/package.json
index b05a1116..a4f95ed9 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "gpt-ai-assistant",
-  "version": "4.5.0",
+  "version": "4.6.0",
   "type": "module",
   "scripts": {
     "dev": "nodemon api/index.js",
diff --git a/services/openai.js b/services/openai.js
index 6363cad4..9f7f0181 100644
--- a/services/openai.js
+++ b/services/openai.js
@@ -15,10 +15,11 @@ export const IMAGE_SIZE_512 = '512x512';
 export const IMAGE_SIZE_1024 = '1024x1024';
 
 export const MODEL_GPT_3_5_TURBO = 'gpt-3.5-turbo';
+export const MODEL_GPT_4 = 'gpt-4';
 export const MODEL_WHISPER_1 = 'whisper-1';
 
 const client = axios.create({
-  baseURL: 'https://api.openai.com',
+  baseURL: config.OPENAI_BASE_URL,
   timeout: config.OPENAI_TIMEOUT,
   headers: {
     'Accept-Encoding': 'gzip, deflate, compress',
diff --git a/utils/generate-completion.js b/utils/generate-completion.js
index 8e0de0ff..bbfe030e 100644
--- a/utils/generate-completion.js
+++ b/utils/generate-completion.js
@@ -1,8 +1,6 @@
 import config from '../config/index.js';
 import { MOCK_TEXT_OK } from '../constants/mock.js';
-import {
-  createChatCompletion, createTextCompletion, FINISH_REASON_STOP, MODEL_GPT_3_5_TURBO,
-} from '../services/openai.js';
+import { createChatCompletion, createTextCompletion, FINISH_REASON_STOP } from '../services/openai.js';
 
 class Completion {
   text;
@@ -22,6 +20,11 @@ class Completion {
   }
 }
 
+const isChatCompletionModel = (model) => (
+  String(model).startsWith('gpt-4')
+  || String(model).startsWith('gpt-3.5')
+);
+
 /**
  * @param {Object} param
  * @param {Prompt} param.prompt
@@ -31,7 +34,7 @@ const generateCompletion = async ({
   prompt,
 }) => {
   if (config.APP_ENV !== 'production') return new Completion({ text: MOCK_TEXT_OK });
-  if (config.OPENAI_COMPLETION_MODEL.includes(MODEL_GPT_3_5_TURBO)) {
+  if (isChatCompletionModel(config.OPENAI_COMPLETION_MODEL)) {
     const { data } = await createChatCompletion({ messages: prompt.messages });
     const [choice] = data.choices;
     return new Completion({