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.
The gameplay would be broken as it is now if there were 6 party members. Larian would have to completely remake every single aspect of the gameplay to accomodate 6 party members. Even the terrain does not accomodate more than 4 at times. All the fights would be severely broken and ridiculously easy with 6.
It is an enormous task to ask Larian to rework the game to have 6 party members.
Maybe when the game is completed, but when they are still making the game 4 party members is enough. Unless you want the game to be delayed and delayed and dealyed some more.