I am saying. You pick a class. There are 4 people in the server with you. A plugin, "autojoin" per say, would allow players that have already selected a class to enter an arena.
So Players A,B,C,D have picked their classes. They will all be put in a game together.
Now lets say we have 4 more players join.
Those players choose there classes and get to join their own arena through the auto join plugin.
Its basically like in COD automatic matchmaking where you join a playlist and it automatically starts game and places you in a certain team. The point of the 4 player limit in each arena is so that it keeps a Super Smash Brothers feel because only 4 players are allowed in one arena.
^Another idea playlists. Team Battle Free For All maybe a challenge arena like destroy the targets.