I have heard a rumor of this happening in the future, the other two players will sit on the side of the vehicle ond be able to fire their own weapons. But this is a rumor and hasn't yet been confirmed by gearbox.
update: The only 4 seat vehicle is the Lancer which can be found in the dlc The Secret Armory of General Knoxx.
Not only is the Lancer a 4 seater each seat has a weapon, Driver : Standard Forward Machine Gun, Gunner : Energy weapon that fires similar to longbow grenades but deals shock damage, Mine Layer : Fires rear mounted HE mines, Blast seat : Emits a shockwave around the vehicle
it is also a quests reward for the quest OMG APC! from Scooter