I don't know why this is such a thing. Four seems fine in my book, and I mean, to get to 6 players wouldn't they have to retool the difficulty on everything. Make a choice, you don't have to play all the classes at once.
For the hundredth time, concerns about "keeping the balance" in an early alpha where no encounter can be considered finalized and set in stone are laughable.
What's worse, these probably come from the same people who think that there's absolutely no problem with asking for four different difficulty levels in the final game, but god forbid if there's a group (a fairly big one) that wants to control a larger party. I mean, the dedicated thread is just the most popular in the entire feedback subforum by a landslide, after all, dwarfing everything else.
And that's without taking into account the dozen of clone-threads talking precisely about the same topic, such this one.