The fourth man (4)
I believe the answer is:
seth
'the fourth man' is the definition.
(fourth man after Adam, Cain and Abel)
This is all the clue.
(Other definitions for seth that I've seen before include "Adam and Eve's third child" , "Younger brother to Cain and Abel" , "Early man" , "A Starkadder" , "Adam and Eve's third son" .)