in our league we now allow EMs even if they are not four player. A single player really results in a couple of the other players walking away until their turn, but a two player typically keeps all of them around (and certainly the second player). Not sure why, but that seems to be the behavior.
There are some two player games that maintain state between players, which for some games is cool, not aware of any four player games that do that.
I've also seen a two player game (Derby Day) where the actions of one player can effect the scoring of the other player, which creates some interesting strategy/skill for an EM.
No doubt a four player game is harder to work on then a one or two player game, but once you have it working right, it is typically not a big issue.
I suggest getting a one player game, a two player game and a four player game. That way you have it all covered