Know it has been said before, but it needs to be six. Stay consistent to the IP Larian. Oh right you only want the IP for window dressing.
The idea smaller smaller makes choices matter more, no it doesn't It makes the party more cookie cutter. The limitation means people will select more often, only the top performing classes, rather than adding classes that don't perform as well for fun.
With the 4 limit that pretty much means unless I'm play a cleric myself (which isn't very likely) that that one slot is always filled. So the only factor is am I playing a melee or spell caster myself on the rest of the makeup. If I'm a melee that means 2 spell casters for CERTAIN. If I'm playing a spell caster that means most likely the same, maybe I sub in 1 melee, that most like will be a fighter since our cleric NPC fills the role of a thief for locks and traps just fine.
So the smaller party didn't make my choices more meaningful at all. It basically made me not have a choice at all.
I don't want to have to rely on mods for this. It should be baked in and let players decide and players adjust the difficulty as needed as well.