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")