The more I read the comments in the forum the more I get the impression that Larian is basically just going to put D&D skins on a different game so it will look like D&D, but it won't really play or feel like D&D. I guess it's a good thing I can still play BG & BG2.
Tyranny of Dragons: "The ideal party size is four characters." Rise of Tiamat: "Four characters is the ideal party size." Tomb of Annihilation: "This adventure is designed to begin with a party of four to six 1st-level characters" Storm Kings Thunder: "Storm King's Thunder is a DUNGEONS & DRAGONS adventure for four to six player characters." Curse of Stradh: "Curse of Strahd is a story of gothic horror, presented here as a DUNGEONS & DRAGONS roleplaying game adventure for a party of four to six adventurers of levels 1-10." Baldur's Gate- Descent into Avernus: "DESIGNED FOR AN ADVENTURING PARTY OF four to six 1st-level characters" Etc...
You occasionally see 3-7 recommended for tournament type adventure play. But, no matter how you slice it, the adventures that make up the pillars of 5e were designed for play in the 4-6 level range.
BG3 almost certainly went with four because that's what the Divinity Engine was already optimized for, but no one can legitimately propose that a 4 party 5e game won't feel like D&D. 5e party balance just isn't the same as 2e party balance.