M-4

From Memory Alpha, the free Star Trek reference.

M-4 is a mechanical classification for the fourth unit in a series with the term most often associated with computers or robots. Examples of the M-4 designation include the M-4 multitronic unit and the M-4 robot designed by Flint.

It is likely that "M-4" stands for "Mark 4"

[edit] Flint's M-4

M-4 was the designation for a robot constructed by Flint. It served as "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, but Flint proclaimed: "Too useful a device to be without, really" and he built another identical robot in the space of just a few hours. (TOS: "Requiem for Methuselah")

[edit] M-4 multitronic unit

The M-4 was also a failed computer version of the M-5 multitronic unit and was deemed by Richard Daystrom as "not entirely successful". The M-4 was the fourth in a series of multitronic computers (beginning with the M-1) which were designed to run a starship with a minimal crew of just under thirty personnel. The failure of the M-4 led to the design and development of the M-5. (TOS: "The Ultimate Computer")