I haven't seen this documented anywhere, so if it's old news forgive me for reposting it. In mame4all, the controls are actually going to all 4 player inputs at the same time. If you hold the d-pad in different directions when you press select, you can insert coins in the 4 different slots. This way, if you're playing a 4 player beat 'em up like ninja turtles, you can play as all 4 players at the same time! It's weird to control all 4 of them at first, but once you get the hang of it it's loads of fun.
