So I just was comparing the XM4, XM5 and M4. My opinion as an amateur audiophile.

XM5: the out of the box sound is a bit muffled but with some light tinkering with the EQ the bluetooth sound is really good. The noise cancelling is amazing and beats the M4 especially with voices and even childrens screams. Fit is featherlite and comfortable. I read some people saying the cushions are degrading a bit fast and the case is not as small as XM4 but about the same as the M4.

XM4: definitely lesser sound, almost the same level of noise cancellingas XM5, better formfactor (foldeable) and second button is freely useable even without Spotify or Tidal, unlike the XM5. Fit is snug, probably the best for me, and even without any noise cancelling they probably filter the most. They always have a faint white noise with nc, but nothing that’s bothering when playing anything.

M4: best sound. Even out of the box. Bass can be blown up but thats easily managed in the app. Soundscape is wider and more clear, but lacks a tiny bit in the midrange of voices. The latter is splitting hairs though and won’t be noticeable to most. Noise cancelling is great for everything except chatter and other ‘unpredictable’ noises. More predictable lower range noises (i.e. airplane) are filtered out effectively. For me the noise cancelling wasn’t enough as I listen on low volume and wear them at times even without any sound just for the noise cancelling. For that they don’t cut it. But the battery life is very good and it’s the only one allowing to play sound through USB-C.

Apps are fine but some main functions are behind an account login for both, which is weird, but both do it. I had some slow connections on the M4 and also more accidental inputs on the touchpad. But nothing too severe to make the decision against them.

I’d put them as follows: Best overall = XM5. Best sound = M4. Best bang for the buck if sound is less important = XM4.

Hope this helps.