Mods can support 4 players, Larian has tried that and said it was fun. But the base game does not support 4 players for story reasons.
That's really unfortunate... Is there some specific reason they can't let players control the followers? The dialogue or something?
Indeed. 4 players co-op would mean that two players can't engage in dialogue.
But if that's no problem for you mods will soon deliver on your request, I'm quite sure.