Yes, you can sharpen m4 on a sharpmaker, reprofiling will be a hassle but that's true for the sharpmaker with almost any steel. I find m4 to be about the same as s30v when it comes to sharpening, maybe even a little easier.
In terms of the m4 ritters, you'll have to find one on the secondary market and they usually command pretty high prices ($200+). There has only been 2 batches, one was several years ago, and the second was this summer and consisted of less than 100 knives (73 I believe). The second batch sold out in a matter of hours.
With that said, I'd jump on that m4 regular grip from gpk. That is a fantastic deal for m4 and the ritters will cost you a premium provided you can find one for sale in the first place.