My R4M is a great camera. I bought mine new!
I wish that I had bought the R4A instead, but that's neither here nor there.
It's a fantastic wide angle shooter, and will work fine with slower 50mm lenses.
I've primarily used it with a 28mm and a 35mm, and have never missed focus despite the low-mag finder. Being able to shoot without those horribly distracting accessory finders is a delight.
The viewfinder is fab and the rangefinder patch never flares. The meter is great!
Film winding and loading is a breeze. The only thing that I have to say about the camera is I don't like the placement of the strap lugs. It causes the camera to hang funny if using a conventional strap. If you're using a wrist strap up some sort this is, of course, a non-issue.
The main differences between a the R4M and a Leica would be the swing back for easier loading, a light meter with more information, a stiffer, one-part advance lever, manual frame-line selection, a shutter speed of 2000, and useful flash sync of 1/125. It's also lighter, and can be picked up used for a fraction of the price of any M.