I mean I may be wrong, but I seriously doubt at this stage they are going to change party size. Because then they would have to totally redo all the engagements difficulty and the mechanics behind them. To most people, 4 is fine. I mean who knows, maybe they will add more party characters, but considering it only takes a second to port to camp to swap out, and the extra work that would be involved in basically redefining the entire set of combat mechanics, I just don't see this happening. Not to mention, most RPGs, 4 is the standard now. It was the standard in their previous games for DOS as well.
Companions will be locked out according to Larian so there will be a point in time you can’t just swap them out.
Four is only the standard due to the popularity of console gaming. Devs are always thinking how they can port their PC games to consoles.