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.
This is it. I think they want to build on the origin story style of PCs that they had in D:OS2 which leads to a very detailed approach to party members. Which is why I'm worried that there wont be more than like 6 or 7 viable party members that you can recruit. As you say, introducing more characters would increase the workload and the game would be released later. Or it could be a philosophy descision, I cant really speculate.
But I will say this, the reason why Minsc, Edwin, Viconia, or any other highly memorable character from the originals are so iconic isnt the volumes of algorithms of pathways for their personal journey.