Got my MX Master 4 a few hours ago, replacing an MX Master 3S (which will move to another machine).
Best change?
No more of that soft-touch coating, which feels great at first but rapidly becomes "sticky", is effectively un-cleanable, and eventually starts coming off. The new micro-textured plastic that replaces it feels good.
Beyond that ...
I prefer the new button for gestures over pressing the thumb rest, though if you have small hands/a short thumb, you're probably going to have to adjust your grip to reach it.
The haptics are interesting; it seems they're the only part that requires the Logitech software after initial setup.
However, the "plug-ins" that let them work with applications require a Logi+ account to download - which is extremely lame (tired of devices that have artificial requirements to have accounts).
And ... at least with the Photoshop plug-in, I can only get one of the haptic features to work ("generative fill complete" - which isn't very useful) and the ones I might find useful (such as "snapped to alignment guide") don't work at all for me - so either something is off or I am not understanding what they're supposed to do.
Not sold on the Action Ring stuff so far - time will tell there.
Daft and cheap that they didn't include a cable - most are a bit stiff to use while charging wherethe one with the 3S was about perfect.
The Logitech software won't be staying if the haptics stuff isn't useful/working properly in very short order.