As for contriving a narrative reason to limit to 4, it had better be a VERY good reason, because if it's not leaving people in camp will still come across as narratively profoundly stupid.
Indeed it is. And every time you come across lines like "it looks like your party is full at the moment" the profound stupidity is rubbed in again. But is this really a problem for Larian to solve? The lack of explanation for fixed party size is a problem for all CRPGs. Shouldn't some organisation come up with a solution that can be applied to all present and future fantasy RPGs? For SF RPGs the solution can always be that your planatary landing craft only has four seats, so that's covered.
