If your's is M-Hexanon 28mm like the one on the right in this link:
I own one and it's output is excellent and the physical/optical quality is on top.
So, you may try to figure out simply by turning the focus and look at the rear of the lens if the cam does move.
As KoNickon wrote to figure out for your M4, if the cam move when you push it.
This lens brings 90mm frame lines on M4, and these must move