As well as what Leon has already said ref the A/M difference, the R4 also has 21mm frame-lines, the only RF camera to do so. The R3 is noted for its 1:1 VF which is great for street work as you can walk and compose with a totally clear view with both eyes while still looking through the camera. Yeah, yeah, I thought. But, until you actually look through a 1:1 VF and move about it is not apparent just how effective this is (useful will depend on what you want to do).
To put the R3 1:1 another way; you can drive a car while looking through the VF. Its that snappy. The R4 is only available in black, the R3 in a few colors including the rather tasty submarine gray.
As to survivability, mine has shot over 6500 photographs since August 2005 and has been rained on heavily, dropped, dragged through woods, snow, & marshes and has only ever needed batteries. Of which, it is particularly frugal.
As per the review, it stays largely mounted with the 21mm Avenon, and perhaps the Nokton 1.4 for low-light. With the native 21mm Voigtlander lens it is pocketable. All-in-all a great machine. Skj.