I borrowed my brother's R4 a few months ago with a motordrive and its nice to work with. I believe the R5 has different metering and that was it.

I loved playing with R4, I found it addictive, it had a the 28/2.8 Elmarit, 50 f2 Summicron and the 135 Elmar. I ran a roll of Tri-x and Classicpan 400 in it and I was happy with the results.