From Lexicanum

Thudd guns (technically known as quad launchers) were 1st and 2nd Edition support weapons. The weapons are four-barreled, firing a salvo of four explosive rounds at a time.

The thudd gun shoots rapidly, its multiple barrels pumping in and out as each shell fires in succession with a distinctive "thud-thud-thud-thud" noise, leading to the common name of thudd gun.

Each barrel is fed by a separate magazine of ammunition, giving the weapon a high rate of fire. Although extremely heavy weapons, they are easily moved due to being mounted on a small robotic tractor unit. The weapon is fired and moved by a radio control box. Due to the simplicity of operation, if the original crewman is killed anyone can take his place.

They are ancient weapons, employed during the Horus Heresy, and even being used within the Imperial Palace on Terra to defend strategic corridors against the attacking Traitors. More recently, they have been employed in the Badab War as well as the Tyrannic Wars.^1-p92

Thudd guns were reintroduced in Forge World's Imperial Armour, where they are known as quad launchers. Forge World's quad launchers are themselves immobile, lacking the robotic tractor unit, but are able to take a Centaur Carrier vehicle as a tow, and unless the weapon is towed it can't be moved.³

Sources

• ¹:Warhammer 40,000: Compendium
• ²:Warhammer 40,000: Wargear (second edition W40k)
• ³:Imperial Armour Volume Five - The Siege of Vraks - Part One