I received M4 yesterday in the second week after posting.
Bass is very strong, but not that much more than my HM5. Treble is clear but not sibilant, fatiguing or harsh. The midrange has taken a bit of back seat, but female vocals sound fantastic. A bit of boom, but not really ear-bleeding painful. It's definitely more mainstream than your everyday audiophile product, IMO.
In summary of my first impressions, I liked the M4 much more than its darker little brother M2. M4 isolates as much as my HM5. Despite the change in presentation, I don't feel like I missed out a lot when I went from HM5 to M4. M4 sounds more stereo than HM5s and doesn't have a deep V shape (as some has said); instead, emphasis in the bass region is slight, generally clear, with a good sense of air and good detail. But I did find the lows were little congested/muddy, which hopefully will go away in time after some burn-in. We'll see about that.
Edit: Forgot to say, the right piece has driver flex but the left one doesn't. It's annoying; it'd be better if both had driver flex during insertion.
