After using the WH-1000XM3s for 2 years and being delighted with them, it seemed low risk to upgrade to the XM4s and get basically the same headphones but with better ANC and multi-device support.
While they seem like an incremental upgrade in every way (and worked great for a little while), I had major problems with static/crackling coming from the right earphone after a few hours of use. I returned that pair to Amazon and received a new pair super fast (fantastic customer service as usual). However, after a few days the replacement pair developed the same issue!
After some research I found that apparently it's to do with condensation building up in the ear cup and interfering with the ANC, which is not properly insulated (wtf sony). The problem seems a bit too widespread for comfort, and coupled with my anecdotal evidence, this is deal breaker for me on a $550 pair of headphones. Won't be trying a third pair.
I learned my lesson though - don't buy things as soon as they're released - wait for other people to find all the issues first.