The OP asked about the Canon 50/1.5 and the Summitar, too. These are beautifully made older lenses, each w/ its own unique character. The Canon 50/1.5 is a sonnar design, and quite heavy for such a small lens. I think it's a lovely lens, and does nice things w/ color film as well as bw. It's much sought after, and not that easy to find these days. And note the comments above about focusing a fast lens on an R4M. The Summitar is a collapsible lens made by Leica between 1940 and the early 1950s (the later versions are coated), and is a precursor to the Summicron. If you're interested in a lower contrast lens that will give you creamy whites, this may be worth looking for. It's soft wide open, but sharpens up nicely around f.4. Check out the "Summitar the Star" thread over on the LTM forum for examples of shots w/ this lens. Note, however, that many older Leica lenses (incuding the Summitar) may suffer from haze, so you should shop carefully and budget a CLA. A Summitar in good condition can be had for around $200 USD these days. A good alternative among older lenses to the Summitar is the Leica Elmar 50 in either the 3.5 or 2.8 versions, which are lower contrast lenses but quite sharp and very good w/ bw. Again, good copies can be found well w/in your budget, and would work well on an R4M.
You also asked about the Summaron, but that is a 35 lens.
I agree w/ Juan, that if you've already got a 35, it doesn't make a lot of sense to get a 40 instead of a 50.