I think its 4 because of the amount of background, options, dialogue, pathways to play that they are bringing to the table & it would literally be to expensive to go to 6 man party - either that or we will be waiting another 2 years for the finished game. Im an old school gamer too but I quite like a smaller party to manage - not that 6 is bad - 4 gets the balanced party fighter/cleric/thief/mage.
I do think Larian studios listen though so who knows what might happen by the time the game is in its final state - but for me 4 is enough to enjoy.
That's somewhat of a rationalization for a false dilemma because not every member of the party has to have dialogue. If that is the reason for limiting the party to four then fine, limit a party of six to having up to four characters with dialogue and the other two can be silent henchmen who fill an important role of providing redundancy to class and skill sets. Heck, I'd be fine with only one member of a six character team having dialogue with the other four being relatively silent henchmen. Often when I play BG & BG2 I use anywhere from 1-4 custom NPCs with only 4-1 organic NPCs who have dialogue. The only time I ever played with all five non-custom NPC personalities with their native dialogue was the first play through. After that I would substitute 1-4 custom NPCs that I would roll up and then rotate the native NPCs in and out. I really don't care if every member of my party has dialogue as long as I can have an ideal party of six.