Most of multiplayer games nowadays support up to 4 players, or even more. Naming only some: New Super Mario Bros WII, Mario Kart, a lot of party and battle games, and a big etc. I know this could be something a bit hard or tedious to implement but I think with PB's potential and this feature a lot of awesome multiplayer games will emerge.
I don't know if this is something people could like, so I want to know your opinion!
And TingThing, if you're reading this, I think you could look into this for the near future, I have already ideas using 4 players, that, even if I don't make them by myself, probably other will do.