This discussion started from a negative perspective with the suggestion that Larian are restricting the number of characters because of constraints. For example:
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.
I don't believe this is accurate, Larian will have developed dialogue for all companion variants as they don't know what the player will select.
From what I understand (from Swen in 'panel from hell') the limitation of 4 characters is all about building a deeper player to companion relationship. This makes sense to me as with other games I've played where there is a plethora of companions I struggle to find any sort of meaningful connection which detracts from the immersive nature of the game. Obviously there could be other factors like game mechanics but that's speculative.
I would prefer Larian restricts the number of companions (so there would only ever be 3 companions in one play through). I always struggle with which companions to choose! However I appreciate it makes sense to provide more options that can work better with the player class choices/style/preference.
One option is the player selects the 3 companions in the game at character creation and only those will be available in that particular play through.
One key benefit of early access is for Larian to respond to the players views and opinions, if there is a strong argument for more companions I'm sure it will come out of early access.
My suggestion is hold off with preconceptions and give it a go in early access, perhaps you will be pleasantly surprised and the reason for 4 players will become apparent.