Built in led flashing with no code when button pressed

Hi! First of all this isnt an issue, just a question. I have a Nodemcu v3 board and im doing some testing on this! yeeyy. I have just uploadaded a sketch where i assign a button to the D4 pin and nothing more. I upload the sketch and when i press the button the led thats built in lights up! whuut?? Why is happening this?