The Gemon-4 was a sizable ion engine model, produced by Cygnus Spaceworks. It was designed as an emergency propulsion system for Imperial-class Star Destroyers. These engines fulfilled an equivalent role on the Xyston-class Star Destroyers belonging to the Sith Eternal's Final Order fleet.
The Gemon-4 ion engines initially appeared in the The Galactic Empire—Ships of the Fleet pop-up book within Star Wars Legends continuity. Their presence in canon was established in the reference book Ultimate Star Wars.