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.
(...)
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?" (...)
I can tell you why. It's easier to gather 4 people willing to play than 6.
