When was your M4 manufactured?
My "guess" the new model M4 guns use the same spring plunger washer as the M2...but that is just a suspicion on my part https://www.midwestgunworks.com/benelli-m2/washer.html
My well used M2 has a washer from a hardware store collection after I lost the original part late one night. Thousands of rounds later I've never seen the need to replace it.