Playing two player right now. Me: A fighter Player 2: A Ranger
we pick up Cleric and Mage companions and then run into the Rogue aaaaannnddd.... we would have to kick out the mage or cleric to get the rogue. I felt a lot more dejected about this than maybe I should have, but not having a rogue, cleric and mage in the party seems like a fail party and yet we can't have all three unless we made our char as one of them. It just feels way too limited.
So yeah, give us at least the option for 6 in a party. If as OP says, you balance the game around 4, thats fine, assuming you put in more than 1 difficulty setting, people with 6 can just crank it up to make up for it.
Also pointing out that there is pretty much no reason not to just add 6 in as a feature (if not the default) because obviously modders are going to make it a priority to get 6 in if Larian doesn't; so if you know it is what a significant number of people want, then just let us have it.