I'm hoping this game is not just a great successor to the BG series, but the best D&D game ever made and THE gold standard 5e game. I just don't know very many DM's that cap their party size at 4, and I would be hesitant to play at their table if they forbid it. DnD is a shared storytelling experience. The interactions between characters and the richness of their stories is a major element of that tabletop experience - and was a quality of BG that helped evoke that feeling of tabletop DnD's depth and breadth within the game.
I want to play with a bunch of different character/class combos. Page 83 of the DMG says that the ideal party size is 3-5 players. Most of the official adventure modules are written for 4-5 or 4-6 players.
Why restrict us to the low end of the standard WotC DnD party size? The only answer seems to be additional effort (and time) on Larian's side, both to create the characters/dialogue and to update the Divinity framework to allow it. I hope this is revisited during Early Access and beyond, because it feels like a Larian-imposed restriction not found in the sources they're building on top of.
