The instructions in that thread linked above seem fine for the P3 but I could not get them to work for the P3 4k.
Wifi reset to default is covered halfway down page 27 of the P3 4k user manual here
To reset the Wi-Fi password on the DJI Phantom 3 4k after starting the remote control and switching on your Phantom
1) Hold down the C1 and C2 buttons and at the same time press in the camera settings wheel button (top right hand corner of your controller).
2) Let go when you hear a beep
3) Start toggling the flight mode switch P - A - F backwards and forward and the status indicator (that is the LED to the right of the four led battery display) will blink red and green, going to solid green when the job is done.
4) You may have to restart your aircraft depending on your exact firmware so do it anyway.
Hold down the C1 and C2 buttons and at the same time press in the camera settings wheel button (top right hand corner of your controller). Its not beep i have tried it 40 times. Whats wrong? Plz help me