As far as i know (and i would dare to claim that i do) nobody ever asked that game should be reworked to disable 4 member party.
So ... you mind even that option? And if so ... why?
That's easy. A party of 6 would make rework of the encounters necessary, despite what friends of the party of 6 say. I'm not sure a proper balance for the party of 4 play would result, I doubt it. You cannot simply take DnD rules and experience because the monsters and combat rules are changed in BG3, what many mourn about. So, although I'm usually always for more options and variety, here I prefer to be very egoistic and say, it's good as it is.
BTW why a party of only 6? Because it is in the DnD rules? Why not a party of 8 or 10, for the lovers of variety. Many more interesting characters and classes to enjoy. It's just a little bit of adjustment, seemingly.