Congratulation, I did the same route - from Nikon to Leica R. I do sometimes notice the difference between the lenses, especially wide open. The build quality is the same - Nikon is good! Your Summicron-R 50mm is among the very best lenses, you can shoot it wide open with confidence. I do find the camera (R4) too small for my mediu-sized hands - it's the size of an OM-1 ater all - so I did get myself a MD with a grip. There are several grips by the way: the R4 grip, the R4-R6.2 and the R7 grip which is a bit longer to accommodate for the taller R7 body an therefore too long for other cameras. The quality feel of the R cameras is not comparable to a Leicaflex or an M Leica, but hey, the are small quiet and disposable.
Enjoy it.