4-0 (pronounced "four-oh") was a slang medical term used during the 23rd century to describe a patient as being in very good health.

Doctor Leonard McCoy used the term to describe Charlie Evans condition, following his arrival aboard the USS Enterprise in 2266, as being "4-0". Charlie was confused by the term, causing McCoy clarify it as "100%. Sound of wind and limb." (TOS: "Charlie X")

The term was also used in the first draft of "Where No Man Has Gone Before", when Dr. Piper (at the time simply the "Ship's Doctor") read off Gary Mitchell's (at that time Clark Mitchell) stats off the body function panel as "perfect", Mitchell restated the Doctor's notations as "Four-oh in all departments, right?".