Some have already mentioned these, but here's my 2 (ok, maybe 3.5) cents:
- The M4 has a less-cluttered VF, with fewer framelines. I like this when I'm composing my shots. I don't miss the 28mm framelines, which are pretty hard to see on the M6 anyway (not to mention less than accurate along with the other framelines). On the M4, I just use the entire VF image when composing for 28mm.
- Unless you get the MP upgrade, the tendency for the RF patch to flare and washout on the M6 can be annoying.
- It's nice to know you have a built-in meter on the M6, especially if you want to just grab and go and not deal with a grabbing a separate meter. However, the M6 meter takes some getting used to--it's more of a large spot meter, and if you're not careful you can make erroneous readings. I find that I'm more used these days to carrying around a small handheld meter which I'm quite familiar with, and I only use it occasionally, eg under new lighting conditions, or when conditions change. The rest of the time it stays out of the way of my shooting. With my M6, I often have to second-guess the readings with every shot. And red LEDs in the VF can be distracting.
- My M4 has that nice balance of smoothness and solidness of operation, more so than my M6. The shutter release is much smoother and quieter.
There have been a few CLA'd M6s with MP VF upgrades on sale lately, so this might be the way to go if you're really set on a built-in meter (which you can always disable by removing the batteries). A little spendier than a stock user M6, but not quite as spendy as an M7 or MP.
Ming