My fear is that they are going to make the same mistake NWN made, which was to designing a game for the Single Player = Single Character (Party of 1) experience, and where multiplayer is really just Co-Op for 2-4 players each controlling a lone single character.
Multiplayer in BG3 allows you to control multiple characters. If somebody drops the host can re-assign that character- so you can have 2 players controlling 2 characters each.
For the record I don't care if we have 4 or 6 characters. I can already defeat the game with 2 players using strong builds so the idea of balance at this point seems irrelevant.
4 characters, 6 characters, 8 characters, 10 characters. All fine with me.