Is it possible to upgrade Gen 3 to use a raspberry pi 4?
Welcome to the forum.
Yes absolutely. Its been designed specifically for this and the RPi4 fits into the same casing. Only thing you need to do is to flash this image on the SD card:
This is our buster image and is required for the RPi4.
So there is no need to any adapter? Everything will work?
No need for an adapter. Everything is compatible with the RPi4.
Does the LAN port work as standard if using your RPi 4 image too?
The ethernet port works on the RPi4 edition. So you can connect your network here, if needed.