The fourth and final class is Support, and it sounds pretty much like it is. Even without unlocks you'll be loaded out with an ammo crate and a powerful machine gun. Dropping that ammo crate for your squad refills their ammunition stocks, even RDX packs and grenades - but be careful to place it so that it only benefits your team, and not the enemy.
Despite the Support class' importance in a squad, their early unlocks aren't as useful as they are to the other three classes. Deeper into their career, you'll find some very cool toys like mountable turrets and infantry detectors, but you'll need to commit yourself to being a dedicated Support soldier if those sound enticing.