The unbalance doesn't have anything to do with the XP you get. Even though the 4 characters will get more XP than 6 characters, becoming stronger individually, the encounters are the problem. The initiative turns become much more advantageous if you are 6 compared to 4, no matter how powerful the characters are. Cause with the system in place right now, your chances to get consecutive characters in initiative order and thus being able to play whoever you like to get an edge in combat would be too high.
And there is another problem with relying on XP to balance the game, it would mean no characters in a group of 6 would ever reach level 10 by the end of the game, and so many players would complain their favorite chars were too weak until completion of the adventure. Tricky to balance this.
The more room you leave between full group number and lonewolf playthrough enjoyment, the crazier more difficult it is to balance.