I use the R4M as my second body - to just about anything else. Usually with a 21/25 on it. Always along - and I mean always! It is light enough to sit in the shoulder bag without having you list heavily. In "emergency" you can use it with a 50mm - finder frames are small, but works. Good meter too. Mine is one of the first Cosina made and it has been used heavily over the years and never missed a beat. If you primarily use wides and "semi-wides" it is an essential piece of kit in my opinion.
Today is a typical example, I took a Nikon SP out with a 35f2.5 - but used the R4M with a Biogon 21mm f2.8 instead. One frame with the SP, 3/4 roll with the R4M!