Why four servos instead of just three? You could still have a square surface with three, just mount to the glass underneath and maybe have a frame on the glass to provide the triangular mount points. That way each servo is truly independent, right now you only have 3 distinct degrees of freedom anyways so you've essentially made an "overdetermined system".
Well, no, it wouldn't take any extra precision. If anything, the current setup takes more precision as you have to keep all servos in sync or it'll be pushing one servo against the others. Right now you could unbolt one of the 4 servos that are on it and it would still work just fine. You wouldn't need to reprogram anything, just having 3 working servos and one completely disconnected would still keep it in the exact same position.
