Ben Dover, CPM M4 is a little easier to sharpen ( subjective , as easy can mean a harder steel, that is more wear resistant but doesn't bur compared to a metal that has almost no wear resistance, but burrs like a SOB.) S90V needs more time, and strokes on a sharpmaker, but diamonds cut all. Even monster vanadium super stainlesses.
Now, having a M4 pocket knife I believe that there is a place for both steels. There are things that they each do that the other one doesn't. M4 isn't much more difficult than M2 to sharpen. In pocket knife settings, I believe that S90V still has the edge in wear resistance.
I like it, as I like M2, but I don't see it as better for pocket knives than ZDP, or S90V. The toughness isn't really needed untill you start to get into jobs only fixed blades are suited for.
M4, like M2 really responds well to the strop. I love carbon/alloy steels but I still would chose a ZDP blade over a CPM M4 folding knife for daily carry. Hard use camping might see the reverse. Joe
Edit: When they have a CPM vanadium monster super stainless that RC's at 63 or 64 like ZDP easily does than I'll call it a true improvement over S90V, and CPM M4 both , and probably ZDP also. Joe