My personal take on this:
Your best choice is an R3M.
It has the ideal VF for the 50mm lens and is a joy to use with this FL. Very comfortable, with just the right amount of "meat" outside the frameline area. If your focusing eye is the right one, you can even use the camera with both eyes open. Of course, it has no framelines for 28mm, you will have to use an external VF with scale focusing + DOF (quick + comfortable), but this is no different from the R2M. Excellent external VF available from Voigt, Zeiss, Leica etc ...
Why would I recommend against the R2M in your case?
Because it is a bad compromise. Compared to the R3M, you loose on the 50mm front, while you gain nothing on the 28mm front. I found the visibility of the 35mm frame lines on my R2A to be good, but somehow "imagining" the 28mm VOF would have been a PITA for me. So you need an external VF for the 28mm. The 50mm frame lines are noticably smaller than on the R3M, certainly workable, but also certainly not as enjoyable and comfortable. I think the R2M is a good camera for the 35+50 combo, but a bad compromise for 28+50. The raison d'être for the R2M is the 35mm frame lines. If you don't own a 35mm lens, you don't need this camera.
The R4M: Only buy this camera if your main FL is 28mm and you plan to do a lot of shooting with a fast lens or/and at close distances AND you use the 50mm lens only occasionally. This is the only situation I can think of where the R4M makes sense, because only then what you gain with R4M (the parallax correction and rangefinder focusing ability for wide angle lenses) is of real importance. OTOH, if you plan to shoot mainly street, scenic stuff or landscapes at infinity, simply pre-set focus + DOF and an external VF for framing is all you need. The 50mm frame lines on the R4M are workable but not really fun if you use your 50mm more than occasionally. You also loose focusing precision with the 50mm wide open/up close, which is normally much more important than focusing precision with a 28mm lens wide open/up close. The R4M is a camera for wide angle nuts or a camera that can complement a set-up as a second body for WA lenses. I don't think it's such a great camera if you shoot a lot with 50mm lenses.