The opinion of the obsessed with 20 year old installment in a franchise group on party size is completely and utterly irrelevant. This game will have a player and 3 companions.
While not exactly cRPGs the current popular turn-based games did use more then 4 units for their combat encounters. XCOM1&2 have 6 units, popular long war mods expand this roster, Phoenix Point supports at least 8 units as well.
At the same time, it also comes down to intended complexity of encounters as well as interesting decisionmaking available within those characters. I felt 6 man limit in FiraXCOM1 was too low, but I didn't feel that with XCOM2, where redesign of classes made them more interesting to use. Also there are stellar turn-based games with less then 6 units. Into the Breach is rather excellent, though small scale, and it has 3 units.
But the biggest mistake I make here, is that I look at it as a single player game, while BG3 is coop. So it's not really a party of 4, but slots for 4 coop buddies, which in this context is plenty. Playing through D:OS1 with only two characters under my control felt like plenty as focus is not on deep, tactical thinking and coordination. That's also a "mistake" many of previous posters make as well - thinking of BG3 in context of single-player RPG. Larian RPG just isn't the same genre as BG1&2, Kingmaker and PoEs.