The AT4 is a rocket launcher used to take down aircraft such as Death Gliders and Wraith Darts.
Major Evan Lorne engages a Wraith Dart
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.