I believe the reason for them sticking to 4 characters is that their engine probably can't handle more than 4 players at once in co-op.
The good news then is that it wouldn't need to. It's not like expanding the party size limit would make mandatory to give each character to a different player... Or to fill the party in the first place.
True, but I don't believe that's how Larian thinks. They want to put huge emphasis of their marketing on "Play a full party with your friends!" And they probably think people would get too easily bored in co-op if they had to wait for 5 other characters as well.
Just imagine this conversation between Larian and the player:
Larian: "You can play with your friends in co-op!" Player: "Wow, cool! Party size is 6, right? So I can play with up to 5 friends?" L: "Well... No. You can play with up to 3 friends, though." P: "But that doesn't make sense. If I can have 6 characters in my party then why can I only play with a maximum of 3 friends?" L: "Because our engine SUCKS, but we don't want to tell you that." P: "Well, only allowing 4 player co-op in a game with a party size of 6 kinda makes it obvious." L: "You are right. We probably should have sticked to a party size of 4 so we wouldn't have this conversation in the first place."