All - jumping into this string a little late, but: I have owned an M4-2 since I bought it new in 1979, it has had one CLA and no malfunctions despite heavy use. There are some design differences internally from the M4, but nothing too critical - steel instead of brass in some of the film advance gearing, an aluminum st&ing instead of an engraved steel exposure counter, no self-timer. As I have said elsewhere on this site, I hope this camera continues to be ignored or even despised by the bulk of collectors - it keeps the prices reasonable.
My usual combination is the M4-2 and the M4 or M3, with a Luna-Pro or MR-4, or when it's just one camera - my M5. While you can feel the gearing difference - just a tiny bit less smooth in the -2 - the cameras are much the same.
Also - I agree about the MR-4 meter unfortunately. Though very handy and quite accurate, most are in pretty rough shape now. Batteries are available, but expensive. The "converter" solution - a sort of hollowed-out PX625 body with a diode and an S-76 battery - works well - I have been using them for about a year with great success. Still, if you're shooting 'chromes and need the metering accuracy, a handheld that uses a readily available battery is the way to go.
Max