See M-4 multitronic unit for the failed computer unit by this same name.

M-4 was the designation for a robot constructed by Flint. It served as his "butler, housekeeper, gardener, and guardian." The robot hovered in the air and was capable of firing a type of heat beam. It was destroyed in 2269 and immediately replaced by Flint's spare M-4: "Too useful a device to be without, really". (TOS: "Requiem for Methuselah")