I have no issue with a core 4 person party as long as I can hire additional mercs, tame beasts, have NPC’s for specific quests join, etc...
If we have enough free slot to hire merc or beast, I prefer to choose a "real" companion. For me, the real pleasure in this king of game is to create a party I love and to interact with them. Discussion, banters, conflict ... Mercenaries doesn't have a personality, they are boring ^^
You have that in spades with the party of 4, Larian are clearly very focussed on the party interaction, but does adding 2 more to that dynamic warrant the extra complexities? Or would we end up with random conversations out of nowhere that have little bearing on what has happened before? It’s certainly easier to tailor the experience more with a firmed up group at a certain stage. I would rather have a more interesting party story where each conversation is relevant and growing to how I play, rather than a larger group with odd interactions that plague other larger group rpgs for the sake of banter.
Don’t get me wrong, I like a party of 6, but there is always 1 or 2 within such a party that get the short straw. Just playing a little devils advocate here.