The Fourth Sleeper was the most powerful of four robots designed in Berlin by Nazi Germany as agents of destruction.
After World War II, the Fourth Sleeper was entombed within a crypt that was sunk into the sea.
The first three Sleeper robots were activated by agents at a certain time in European villages, consisting of a giant human-like robot with blaster rays, a winged robot, and the 'brain', which resembled the Red Skull and was a powerful bomb.
Despite Captain America and the personnel of a nearby US Army base's attempts to stop them, the three Sleeper robots combined and flew towards the North Pole.
While in pursuit with the military, Captain America surmises that the first Sleeper would use its blasts to dig into the ice, enabling the robot to travel into the Earth and explode, destroying the world, as the Red Skull had vowed that if he could not conquer the world, he would destroy it.
To prevent this, Captain America is able to board the combined vehicle in midair and detonate the bomb early with a flamethrower, destroying the three Sleepers.
The crypt containing the Fourth Sleeper was retrieved from the sea in modern times, and when the Sleeper reactivated it destroyed a seacoast smelting factory and battled Captain America.
The Fourth Sleeper was ultimately rendered intangible by a "sonic crystal".
Similarly, the Red Skull later activated a Fifth Sleeper, only to have it stopped by Captain America as well.
The Fourth Sleeper was later restored to tangibility by the Machinesmith and was used as a "Trojan Horse" to gain entrance onto Avengers Island in order to liberate the various robots incarcerated there. It was thwarted by Captain America, and badly damaged.
The damage done to the Fourth Sleeper was later repaired by the Machinesmith. The Sleeper was animated by the Machinesmith to join the Skeleton Crew in search of the missing Red Skull. It battled Hellfire Club mercenaries, and the Black Queen. Replicas of the five Sleeper robots were then pitted against Captain America and Diamondback.