The R4 finder is a very complex piece of work! It is not just a lower magnification finder with some wide frame lines stuck in it. In Rf designs it is a first of its kind and it took almost three years to develop.
It is obviously designed for the wide angle user - the 50 finder is small and "peepholish" and is more of an 'emergency" finder.
If you are a shooter who primarily uses 35/40/50/ and longer lenses it is not for you. I am wide-angle shooter and the 35 is my normal and the 50 I treat as a short tele! The R4 is a dream come through - no more top-mount finders to loose or snag in straps and a faster frame/focus than with these finders. I have found that my "hit" rate (keepers) with 21/25 lenses has vastly increased with the R4. Shifting from one finder for focus to another for framing breaks the concentration while shooting and with the R4 i dont have that problem. The frame-lines are straight and clearly defined though the 21 lines are right at the edge of vision with glasses, but no more so than the 40 lines on the R3 or the 28 lines of a 0,72 Leica M2. I am considering the R4 as a "mile-stone" camera, right up there with the first M3/M2's with the bayonet lens-mount and the Nikon F. I have had mine since mid-March and so far I have never ventured outside without it! I usually match it with a M2 or R3 (and the 40/1,4).