your system is well done, anyway, i will add another that i've made that looks as yours.
and a critic aout yours
(well, if you press the wrong ones, after battle if you touch the door, they will appear anyway. should reset after.)
take a look on this one. it's almost the same: (just 2 pressed are the correct, or a mob will rise, and close the door.also resets the switches after battle.)
anyway, to who want to take a look:
http://www.megaupload.com/?d=5MIOSP4Swell, it's a good system, and i posted mine since it is about the same thing, and wouldn't be needed another topic for the same thing.
KeEp the Go0d work =)