The AT4 is a rocket launcher used to take down aircraft such as Death Gliders and Wraith Darts.
They have been used by both the Atlantis expedition and the personnel at Icarus Base to defend against aerial incursions. (SGA: "The Siege, Part 2") (SGU: "Air, Part 1")
Behind the Scenes[]
The M136 AT4 is incorrectly portrayed as a surface-to-air weapon firing a self-guided missile in the series. It is a disposable, one shot, "dumb fire" (unguided) weapon meant primarily as a light anti-armor weapon. Like its Warsaw Pact counterpart the rocket-propelled grenade (RPG), it can be used against slow and low-flying aircraft like helicopters to reasonable effect. It nevertheless could not be reasonably expected to hit fast-moving targets, like Darts and Death Gliders as is shown.
However, it is possible the SGC was using the AT4 to fire a rocket with integrated alien technology and a Naquadah-enhanced payload.