All four are back home (5)
I believe the answer is:
villa
'home' is the definition.
(I've seen this before)
'all four are back' is the wordplay.
'four' becomes 'IV' (Roman numerals).
'are back' is a reversal indicator.
'all'+'iv'='alliv'
'alliv' reversed gives 'VILLA'.
(Other definitions for villa that I've seen before include "where 6 [VERDI] lived?" , "Holiday residence" , "Midlands football team" , "Type of house" , "See 3" .)