Each button has only 2 states - on and off, so basically you have 2^4=16 combinations, just try them all and check after each one whether a wall has opened up in that room. If the room is in it's initial state you only have to press 2 buttons, as you can see in
http://dmweb.free.fr/?q=node/139 , but if you have already pressed some buttons you will have to try out the up to 16 combinations as described above.
If you have further questions you'll probably get quicker response if you use correct coordinates from the Encyclopaedia or from one of the dungeon editors.
Parting is all we know from Heaven, and all we need of hell.