I've looked at this a few times today - very difficult!!
A few thoughts which may or may not be anywhere near the mark:
16ac REWINDS - def = turns back; sub is in 11ac E(ast) Winds (Mah-jong points.)
5d RE-EMERGES - def = come again?; sub in 1d a quarter E(ast) tripled gives 'EEE' which consumes 'merg.
7d RELATE - def union negotiators (IE marriage guidance); ELATE sub is in 22ac - lift.
22ac RADISH - def is root; sub is in 7d a course.
22d REEFER - def is jacket; sub in 15ac 'on price that's been raised.
Think 9ac could be EDUCE - def maybe 'form'.that would give 6d as duplicated possibly.
In EDUCE I wondered if sub for DUCE could be in 18d - actor once = DUKE (John Wayne)foreign gives DUC with E(nglish)
But it's doing my head in to be frank!!
So 3 of the 4 letters could be R, E & D.