For shooting street, I'd definitely go with the R3 over the R4 if only because of the shutter lag. You can speed up the R4s by holding the shutter release halfway down, so it's faster than from full throw, but it's still not as fast as the R3 from any position. The R3 is also one of the most predictable shutter releases I've ever used.

My favorite street camera is my CL with 40mm Summicron, although I like the 1:1 finder on the Bessa R3A better. The CL attracts no attention, spot meters accurately, and is very quiet. The 40 Summicron is a great lens. The side hanging strap also lets it hang in the crook of my left arm as I walk, so it's almost hidden. I once shot a photo of a carney who was threatening me for attempting to photograph him. I hit the shutter and then lowered the camera. He never knew I took the shot.

There are some issues with numbers of lens cams on the SL vs R bodies, but IIRC, the 3 cam lenses work on all but the Leicaflex (original non-TTL metering SLR) body. I'll see if I can find a reference. If not, someone will be along to post the correct info soon. The SL has an overall fine microprism for focusing with larger microprisms in the center circle, and the finder is BIG and bright, really nice. The SL2 has a central split image rangefinder and fine microprism field, 3 stops more meter sensitivity than the SL, and finder illumination for low light.