I just got these.
Sound
Sound is great. My main pair is a Sennheiser HD 600. The Momentum 4 feel like a closed back version of those with a subbass extention all the way down to 20hz.
Bluetooth and App
So far I didn't have any problem. Neither connecting to my phone, neither connecting to the app. The app also works as it should (android).
ANC and Transparency
ANC is slightly worse compared to my now broken Sony XM3. There is also more of a pressure feeling with the Sennheiser. The transparency mode is strange. Even if I'm inside a building with absolutely no wind (not even AC), the headphones produce some kind of strong wind noise.
Build quality
This I can't really talk about. I own some pairs of Sennheiser headphones which are made out of plastic and are still going strong even after 5+ years. So Sennheiser definitely know how to make solid plastic. With Bluetooth headphones, the plastic is more prone to UV exposure, which deteriorates plastic way faster, and I have no idea which kind of plastic got used with the Momentum 4.
They feel better than Sony XM3 and XM5, but worse than B&W PX7 S2.
Comfort
The headband feels good. The ear cups not so much. I have no idea if they are too small, too shallow, or if the shape just doesn't fit my ears. Both ears touch the inside of the headphones, the right ear is slightly worse. If I fiddle around and find just the right spot, they are ok comfort wise, but just ok. And this is too much hassle for me.
I can't really get behind Sennheiser's reasoning for changing the ear cup design. I have two pairs of Sennheiser open back headphones (HD 598 and HD 600), with which I have no problems comfort wise, I tried the Momentum 3 and had no problems either, but the Momentum 4 don't seem to fit.
I really tried to like them. I love the sound (like it better than the Momentum 3), but every time I think about wearing them, I second guess. This isn't a good sign. The Sony XM3 got somewhat uncomfortable after maybe 4 hours, which is normal. The Momentum 4 are uncomfortable from the get go, if they don't sit just right on my head, and still then, my ears immediately feel irritated.