Mail four letters from impostor (4)

I believe the answer is:
post
'mail four' is the definition.
'post' can be an answer for 'mail' (synonyms). I am not sure about the 'four' bit.
'letters from impostor' is the wordplay.
'letters from' says the answer is hidden in the clue (a selection of letters from the clue).
'POST' is hidden in the letters of 'impostor'.
Can you help me to learn more?
(Other definitions for post that I've seen before include "A column" , "Member < to comment on website" , "Send via the mail" , "Mail; office" , "Appointment (with the Royal Mail?)" .)
