Ever since the first announcement of BG3, I have been thinking 5 would be the dream number for a party.
I know 5e gives people enough freedom to get by in combat/story mechanics with 4, but 5 feels better to me -- and not just for the combat system or mechanics (although yes, for different combinations there) but for story flavor specifically. Different companion combinations for personalities and banter shenanigans and interjections. It's just more interesting, and I don't think 5 is too many to be a tight-knit group going through a crazy experience in a big grand adventure together. The story itself, a shared tadpoling vs PC is the special hero going through something unique to them, in a way lends itself to having an extra companion; they wouldn't need some outlandish/random reason to join up with the main character.
Different combinations of companions adds to replayability for a lot of people, but the more limited the spots... I'm not sure how to explain it as well as others in this thread have except to say if it's too limited, it feels tedious to replay for that reason. For example, I have zero desire to go through DOS2 again to experience the two companions I left out of my party; it doesn't feel like it would be different enough. As in, I might play through DOS2 again for another reason, but the companions aren't one of the factors. In BG, the option to mix-and-match companions has always been one of (and sometimes the only) factor in a replay, for me.
And I know, I do get it, this isn't the BG of old and obviously a lot goes into these decisions. I know it could be only 4 for a number of reasons. It could be that 5e is aimed at 4 people so 4 people it is. It could be that we can collect a certain number of companions at our camp and switch them out so it's hardly punishing. It could be that there will be less banter and interjections overall and so we're simply not missing out on anything there. Or it's a limitation related to multiplayer. Or it could be engine troubles. Or a combination of those reasons, or none of them.
Will we be playing BG3 with a party of 4? Probably. Can we do that just fine, no complaints on getting through a fight or story area? Sure. But clearly the opinions vary on the overall vibe and replayability that decision actually gives the game and I do think it extends beyond nostalgia. If I was just nostalgic, I would say six members, but I do agree that would be too many here. 4 seems to be the norm these days, but for me, 4 always feels limiting in terms of story, and a little boring for combat.