Well hello.
Ive thought about this more then enough my self, it would seem to me that you only need a 4 player coin door if your going to be playing a large amount of gauntlet, simpsons or other 4 player games. But wiring 2,3,4 to the same door would put a coin in for all of them, which may be a problem if your not playing with all of those 3 players.
I think that you wouldnt need a coin box, but you would need something to catch whatever falls through, so like a bowl, cardboard box or peice of tupperware.