4v4 is a format of Team Fortress 2 competitive play. It features a 4-on-4 scenario with a limit of one of each class on various King of the Hill maps. It is more fast paced and deathmatch-focused than standard 6v6 competitive play.
4v4 forces one of each class with the only exception being you cannot run Heavy and Medic at the same time. Beyond this any class composition is allowed. Because of this and its sparse weapon ban list, the number of strategies and team make ups that a 4v4 team can employ is immense.
The standard line up tends to be
The Medic is usually a constant, and is only switched for a Heavy when the situation is dire. The Demoman gets much more flexibility for offclasses than he does in 6s, but is still usually run full time. The Scout and Soldier, however, can switch class every life. Pyro and Sniper are very popular and can do a great deal of work when the enemy isn't expecting them.
4v4 is currently only run in UGC and Ozfortress, and as such only has one list of maps. The current maps being played for 4v4 are: