I can understand how the stats would make adding 4 playable characters cumbersome, but it is rather odd how he's not even mentioned anywhere in the game itself.

Yeah, the other 3 busters all use the same palette and identical body, but just swap heads.

I doubt that considering the bosses use various palettes, plus aren't the other 3 using the same color set? Winston would need a separate palette because of his hands alone (been quite a while since I ripped anything from the Genesis game, so don't remember if the others had unique sprites or were head swaps). Also checked the manual, it mentions the game is after the destruction of the temple of Zuul, so at least the manual writer had seen the movie all the way through.

A better theory is they didn't want to add another character to complicate stats with (one is balanced, one has better speed, one has better stamina).

@shadowman44 My guess is palette limitations.

My theory as to why most of the ghostbusters games never added Winston was because they probably only watched the movie up to before he joined them.

@Omega Well you're in luck: There's a ROM hack in the works that'll add Winston in rather than replace either Peter, Ray or Egon.

Baffles me how Winston was not playable in that game