There are a lot of classes, races and subclasses in D&D 5th.
- With 4 party members, having a cleric as MC makes Shadowheart redundant in the party, same goes to wizard, warlock, fighter and rogue. So, depending on the class the player chooses to use as MC, it end up disencouraging the usage of some companions in the group.
And depending on the companions the player likes, it end up limiting the options to make the MC character.
- With 6 party members, there's enough to spare at least 1 spot to try different tactics with different classes, encouraging players to experiment more with varied classes/multiclasses. Not only that, but failing attacks and being knocked down is easier to accept because it's easier to the player to recover from that.
I would like to see how a party with 5 members work first, and depending on the feedback, a 6 members party.
Edit.: Choosing classes, races and subclasses have much more meaning in BG3 because it's based on D&D, than it did in DOS 2 (which was basically a character background that could expand and allow any kind of customization possible - which, btw, would be fine if BG series were based on GURPS, and not D&D). A wizard can't learn a lot of fighter skills, a druid can't learn wizard spells, and so on.
Every class has a specific role in DnD5th, that's why, IMO, only having 4 spots limit players choice.