From what I understand, m4 is a very hard tool steel. Arkansas might sharpen it (I'm not sure of the carbide content...) but it will take forever because of its wear resistance. I'm sure silicon carbide can do it, and diamonds eat m4 for lunch. Found that out while working on my super Freek. The edge was slightly off to one side, so I worked on getting it centered. Took maybe 10 minutes working slowly on 325 grit diamond.
For a straight razor you probably don't want to be that coarse though, at least not for a final edge. I'm not sure of the intricacies of sharpening a straight razor, but I'm sure you have to highly refine the edge to avoid irritation.
Just looked it up... 5.5% tungsten, 4% vanadium. Arkansas stones will do all of nothing with m4.
