It is very easy to differentiate the Su-20 and Su-22M4/Su-22M3. The Su-20 has the fuselage looked like a straight pipe with a narrow "hump" at t its top while the Su-22M4 and M3 have the fuselage at the cockpit area looking like slightly curved down. The top "hump" behind the cockpit is larger and getting narrower towards the tail. Additionally there is the air intake for cooling devices at the fin leading edge on the top of the hump for the Su-22M4. Contrary to that the Su-20 and the Su-22M3/Su-22UM3K series don't have the air intake there.
Su-20 ...
Su-22M4 ...
Su-22M3 ...
Su-22UM3K ...
the pic suece: Samoloty - MUZEUM SIŁ POWIETRZNYCH W DĘBLINIE and the net.