This is different. Lol. Just give players - all players - the ability to have a party of 6. If they don't want to adventure with 6, they can just take 4 with them. No options needed in the options screen. You can literally take as many or as few as you'd like. Just because they give you a max of 6, it doesn't mean you have to take 6 with you.
That is not a good argument. Very few players will want to automutiltate. If the max is six, people will play with six and assume the game is meant to be played with six.
Which is why the default max should remain 4, but there should be an option in game settings to allow up to 6(+?) with a warning that this is not the developer-intended experience.
This is particularly necessary for the case where Larian doesn't change any difficulty aspects of the game for increased party size (the nearly-trivial cost option). If players are by default allowed to use 6 PCs but not warned that the game is balanced around 4, then many players will find the game mind-numbingly easy and likely won't even think of using <6 PCs, let alone actually do it.