I sell plenty of Leica at work.....M4 is meterless and a wonderful mechanical camera, the M6 only requires some battery power for the light meter. There are several versions of both cameras, so it depends on what is available and what your requirements are.
The M6 is probably going to be easier to find and nicer to the wallet as there are still heaps of these around. The M4 will be more difficult as they are older, and good copies are going to be expensive, anything too cheap will be heavily worn. Watch out.
PS have a read about rangefinder magnification and frame lines available....think about the lenses you are going to use and try to find a camera with a rangefinder that suits (there are 0.85 and 0.72 finders...0.85 are rarer and are more suitable for 50/75/90mm)