I am very much a supporter of the 6-man party. I hate viewing D&D though a min-maxing tank healer dps lens. I want to be able to add flavor to my parties and not have to drop pick up companions just to do X because my party is missing Y. (I did treat the copper coronet as a companion safehouse but that was for their story missions) For party make-up I find 4 it too limiting D&D has range of classes that can for fill at least two of the three, if not all 3 of the trinity (cleric, druid, paladin) as well as the option to Multiclass, (level cap of 10 impacts power of any multiclass build) allowing a bounty of options for party creation. My favorite party formations in BG2 where:

Keldorn Paladin or, Jaheria druid/fighter
Yoshimo Thief, or Imoen thief/wizard, or Nalia thief/wizard, or Jan thief/wizard
Aerie: Wizard/ Cleric, or Edwin wizard
Anomen: Cleric/ Fighter, or Viconia Cleric
Minsc: Ranger, or Valygar Ranger, or Massi Fighter
Player: Fighter/ Wizard
I Like casting spells.

My parties in BG1 weren’t much different with me fighter/wizard, Minsc, or, Ajantis, or Kivan or Khalid, Xan, Imoen, and Branwan

Having watched Sven play I assume just like the previous games and D&D in general that Rogues will be 100% necessary and there by will occupy 1 of 4 slots. taking either dps or support roll.
With the three I have left unless I choose to omit either healing or tanking or combined them into a single character like Anomen in BG2
I may have to change or think really hard about what character I will make. Not knowing how many companions or what classes they are/will should be is exciting, this is a new game! It shouldn’t be a point of concern, but I fear only having 3 to choose from. May force my hand into more of a utility role and not the dual wielding glass canon that I want to play.
I find the argument that 6 vs 4 makes for too many turns and will make combat too long disappointing. Chicken / egg. Was the turn based in before the party size I think so, is it a result of play testing? Yes?
Do I want fluid game? Yes. Do I want that at a reduced roleplaying experience no.
But course that’s a single player perspective, I keep reading that party size is the way it is for multiplayer. Is BG3 at its core multiplayer a game?

Tldr: 4 instead of 6 hurts my feels is a direct result of game design and likely not going to be expanded to 6 without crippling the game.