You have to ask yourself, why would I want an M4?
Personally I love an M4, part of it is my personal use tradition, and part of it is function, but an M6 is a fine camera and should do everything you want. I once sold an M4 to get a new M6, it was an upgrade for the meter and I liked and used the camera for 15+ years.
M4:
1. more actuate frames
2. last of the great Wetzlar made traditional cameras: engravings, real vulcanite, brass parts, details like the little machined metal grip on the rewind lever, front windows with beveled frame, crackled black paint inside the bottom plate….
3. finder is old style assembled with full adjustments, not glued together , and a glass condenser lens (not plastic like the recent 'upgrade'), little 'white-out' flare problems
4. self-timer, you may not need it but if you do its there
M6:
1. built-in meter, early version has had problems, not so often maybe, but its one more thing to go wrong and an expensive fix.
2. basically an M4 in design, but with some cost cutting: zinc top plate, no engravings, plastic film counter dial (which is prone to failure, mine did), glued on body covering, flat paint inside bottom plate…
3. glued together finder, if something goes wrong its a switch-out replacement
4. steel geared advance, not as smooth maybe but handles a motor better if you use one.
And maybe the most important factor is if either camera is in good working order. Both are getting on in years and who has worked on it may be just as important as when. Either camera can be a great user.
Here's a couple flavors of M4s
