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.
But, what if they balance for 6 and it's too hard for party of 4?
Difficulty settings. They said we'd get them eventually. If party of 4 is too hard, decrease difficulty a notch or 2.
Again, game is balanced currently for 4, for the most part. So make EA a difficulty setting. Then add a Core 5e setting, which would be harder. Allow for party of 6, and balance 5e difficulty for party of 6. Those who want 5e AND party of 4, be ready for a challenge.
But, again, if they did XP split right, it'd work out anyway because party of 4 would go up levels faster than party of 6.