This is what I did. In mame, I remapped the key for coin slot #2 so it credits p2, p3, p4. It works for ninja turtles as long as you don't press start for the absent players. It works for most other 4 player games as well EXCEPT Xmen. In Xmen, your players start automatically, so if you have only 3 people there will still be 4 on playing, only one will not be moving. You can get around this problem by reassigning the credits.