Originally Posted by GM4Him
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.

Originally Posted by GM4Him
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.
In all likelyhood, difficulty related to the number of characters in a party is not just a matter of setting attack and defense values. One of the nice things about a limitation in party composition is that you will have to make do with what you have. Sometimes you will step into a trap because you did not bring a thief, or you will have trouble healing because you left the cleric at home. So you will encounter more situations where creative thinking is in order. In that way, the number of party slots also influences difficulty.

In another way the limitation could benefit gameplay too: With a maximum of four classes in your party you are more likely to have to rotate people, depending on an estimate of the situation ahead. That, in turn, means more variety in character interactions.

Having to change knives seems more fun than carrying a swiss army knive all the time, so to speak.

For the record, I am fine with four characters, if that is how Larian wishes to design its game.