0 = 4X is or was a male, sentient robot.
The only one of X - Y + 2's creation not to be built out of junk, and to have his own name and identity, 0 = 4X served as X-Minus's right hand man in lair in 1967 while he plotted conquest on humanity. Four-X was very loyal to X-Minus, but not particularly intelligence. After being assigned to guard them as prisoners, Four-X was hoodwinked by Mickey Mouse and Goofy, who deactivated him with oil and then cut out his control-unit (the satellite-dish-like item bolted to his head) to use as a ticket for safe passage through the base. Whether 0 = 4X was later repaired, reactivated, and rehabilitated like his Master was is unknown.
Behind the scenes
0 = 4X was only ever seen in the 1967 story The Case of the Sinister Robot.