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.