I agree with Ken Shipman that the best R4 is an R5. I didn't like the way the R4 displayed the shutter speeds. They were too hard for me to make out in the finder. In the R5 finder, they light up with LEDs--much better. My R5 and R6 also have a diopter adjustment, so I can fine-tune the finder exactly to my eye. That's worth a lot.
The R6 is nice for any of us who also shoot with an M6 or MP (or M7), because it displays the exposure in exactly the same way as the rangefinder models: three LEDs, left, right, and a center dot.
Things I don't like about the Leica R models, compared to my Nikon FA, FE2, and F3: the Leicas feel more awkward to hold and shoot with. The Nikons are more ergonomic. One reason for this is that all other cameras I have used, including the Leica M series, and all my Nikons, as well as Olympus OM series, have the shutter button one inch in from the right end of the camera. For some reason, my Leica R cameras have the shutter button one and a half inches in. This forces me to change my right hand grip in a way that does not come naturally. The camera is no longer an extension of my own hand, in the way that an M2 or M6 is; or in the way that a Nikon F3 or FA is. So instead of the camera being an extension of my own eye, it is now "Rob vs. the Leica R5."
Another thing: the mirror action in a Leica R is a bit sluggish, and the mirror slap is significant. Camera of choise from that standpoint is the Nikon FA.
But, my R5 and R6 have the best damn viewfinders I've used. I can actually see what I'm doing better than with other cameras. And some of the lenses are simply the best: 50mm f/2; 90mm f/2.8; 135mm f/2.8; 180mm f/3.4 APO. (Some of the lenses are just a tossup with the Nikkors: 24mm f/2.8; 28,, f/2.8) So, I persevere with my R cameras. If there's room in the jeep, the Nikons and the Leicas both go. It's an imperfect world.