I thought about this when I made my cab. I ended up with coin buttons for each player. I don't have a 4 player coin door either.
Here is the problem
Games such as TMNT depends on what coin slot you put coins in to give that player credit.
Solution 1:
You can use say player 1 to give credits (shift + down) lets say. You can also use player 1 to give credits for the other players say (shift + left, right ... etc). Problem with this solution is does player 1 want to take time out from their game, most likey lose a life and whatnot to give the other playes credits. Most likely it'll get annoying.
Solution 2:
Use shift and then have each player add their own credits via one of their buttons. Problem with this solution is (with the I-Pac4 at least) is that you need 2 shift buttons because players 3/4's shift won't activate the shift functions on their buttons by pressing player 1/2 shift. You could get around this by useing a combination of inputs I suppose, say some of player 3's wires go to player 1 inputs and some of player ones goes to player 3.
The other problem with this solution is that if someone is adding coins and holding down shift it also screws up everyone elses buttons and causes them to shift as well as long as that person is putting in credits. Also is the shift in a place that everyone can reach it easy?
Solution 3:
I'm not sure if this works or not but I think it will. Have 2 coin buttons, 1 for player one and one for player 2, and anytime someone puts a coin in for player 1, map it to put a coin in for player 3 as well.
Solution 4: Just put a dedicated credit button for each player. You might want to look at the mini buttons. Thats what I used, they work great. If you are worried about clutter but the on the bottom or side of the cp.