Fourth man went down hard (4)
I believe the answer is:
seth
'fourth man' is the definition.
(fourth man after Adam, Cain and Abel)
'went down hard' is the wordplay.
'went down' becomes 'set' (eg the sun goes down or sets).
'hard' becomes 'h' (abbreviation used in pencil classifications).
'set'+'h'='SETH'
(Other definitions for seth that I've seen before include "Vikram --, Indian author" , "In disguise, the brother of Cain and Abel" , "Adam and Eve's third son" , "Old Testament character" , "Adam and Eve's third child" .)