Out of the box it's 2 players only, with intended play-style being each of the main human players is controlled. When I played my co-op game that is what we did, plus we each picked one of the companions to run. You'll also see that when you create a co-op game you can only invite one player.
That said, I believe I've seen mod support for up to 4 players. Haven't tried it and don't know if it works. If it does, I imagine two of the four players wouldn't be treated as "real" characters without some re-coding, and thus wouldn't get to make decisions, get personality traits or talk meaningfully with NPCs. The game is still new though so who knows what will be developed.
