I would think the option to enable a 6 person party in the settings would be best, keeping the default max at 4. I say this because I prefer playing with a smaller party personally. My fiancé and I play 2 characters each. That seems perfect for role playing our own Tav and having a side character to role play as they should be playing based on their character background.
However, when playing DOS / DOSII, we never went the Lone Wolf route. Even though we talked about it many time, every playthrough we ended up deciding to pick up a companion.
Similarly, I'm playing through BG and BG2 right now and guess what...I'm playing alone with a party of 6. Why? Because it's the default and I feel like that's the intended party size for the game, and that I'll miss out if I leave empty slots.
With BG3 though, we play to play through numerous times. The ONLY things that will keep us playing after the first time through is playing a new class Tav, and having different companions.
We've already run through all of the existing companions, multiple times. Even now, just changing Tav to a new class is pretty damn boring because since there are no random encounters, every playthrough is exactly the same, with the exception of new companion experiences.
I personally would rather play through multiple times with new, different companions than one or two times with 4+ each time.
In defence of the game getting boring after multiple runs, this is an incomplete act 1. Even if they expand the party to 6, the full game is going to have way more content. Hell, even act one of the full game is gonna have more content. Not just moonrise tower, but the shadow-cursed region which presumably will be about as big as the underdark. Plus potentially more companions as well, so there's still gonna be a huge amount of content to potentially see even with a party of 6.