I have always been a big fan of Sennheiser and have also had the Sennheiser Momentum 2 before these headphones, but I have to say that I am not happy with the Momentum 4.
First I will mention the good points. The first: the sound quality, the headphones sound wonderful. The depth of the bass combined with clear highs makes for a great listening experience. These are hands down the best sounding headphones I've ever owned. The second positive point is the comfort, with the thick cups the wearing comfort is so nice that I wear it for hours without any effort.
However, not all aspects of the Momentum 4 are rosy. The on-head detection has regular glitches for me. The headphones have the idea that they are no longer on my head at the slightest shift. This leads to clumsy interactions, where I have no control over the headphones and the auto-pause function is more of a nuisance than a help. Phone calls are unnecessarily complicated by the feature's automatic muting, forcing me to switch to my phone. You can disable the feature, but that leads to me inadvertently operating the headphones with my chest when they're hanging around my neck.
Furthermore, the headphones seem to have trouble isolating my voice during conversations. Even worse, it seems to amplify ambient noise, which often disrupts outdoor conversations. Even soft background noises such as keyboard clicks, the turning of a newspaper, or birds in the garden are prominently conveyed to my interlocutors. Because of this I often have to switch to the my phone, or the speaker function that works better.
The build quality, while generally solid, also has a few disappointments. After only seven months of use, the fabric on the top of the belt has already started to peel off. I am currently looking into whether this is covered under warranty, if not I will have to glue it myself to prevent further wear and tear.
Although I have always been a fan of Sennheiser, this product unfortunately did not live up to my high expectations. Given the price range of these headphones, that is a particular disappointment.