Early in the kickstarter Larian was thinking of having 4 player co-op, but the players controlling the henchmen just wouldn't be able to take part in conversations. Then they decided to focus on 2 player co-op.
However, mods can support 4 players, and the main campaign can be modded to allow 4 players, as well. Larian has played the main game with 4 players, and during the recent twitch.tv stream about the editor, implied that mod might get released if they have the time to clean it up a bit, but the game wasn't balanced around 4 players.
