I'm sure the GXR is a fine camera but the M4 is a true classic. It will deliver as good, if not better, results than any digital compact for many years to come. Without being disrespectful to Ricoh or digital photography in general, from a purely economic point of view, the M4 will retain its current value whereas digital cameras' values drop like a stone from day one.
I had an M6 years ago which I traded to part fund acquisition of a Nikon D700. I love my D700 but I had to save up for another M6 (TTL version this time) and I won't ever be selling it unless my wife and I are in dire straits and need food / shelter. My D700 would have already ben sold by then.
I know it's more about utility than economy but I don't know anyone who's sold an M class and not regretted it....