I don't think the M5 is what you want - it tends to underemphasize treble and has tons of bass compared to the M1. Not sure what your price range is and I've never heard the M4 but I think you'd enjoy something a bit more balanced, maybe a Maximo iM-590 or Xears XR120 Pro. If your budget goes up above $50, maybe a VSonic GR06, Rock-It Sounds R-30, Etymotic MC5, or MEElec A161P/Fischer SBA-03. The Rock-It Sounds has slightly more laid-back treble compared to the others but the midrange is nice and forward and it's a surprisingly detailed earphone.
Thanks for your and sorry for my late reply.
I'm from Europe and unfortunately, most of these IEM aren't available here, the only one is the MEElec A161. What do you think about the Brainwavz M3 and MEElec A151? Budget is about 70 € (90 $).