Four Instance
I expect that I'll soon to have giant heaps of praise for MuseScore 4. But presently, after more than two very productive years in MusesScore 3.6, I'm having a fairly dented "new user" experience which led me to post on a number of relatively minor annoyances, and all that has distracted me from actually trusting MuseScore 4 enough to delve and find the joy that others report.
But the real kicker is on opening four documents I get four separate instances of the MusicScore 4 app! And this is a real issue for me, because at the end of a busy day of private lessons I often find 20 or more MuseScore 3 documents open. Navigating the MS3 tab bar was awkward at times, but—even with Mac's Command Tab option for navigating through apps—thus far, I find MS4's multiple instance environment considerably more awkward. At the moment I haven't time to innumerate, but I see many UX costs associated with it. And I've never seen anything like it on the Mac.
I imagine there must have been some huge constraints or rigidities that led development away from creating a normal multi-window interface. Instead MuseScore 4 got the bizarre multiple application-instance scenario with which we now grapple.
I can only hope this is an brief interim state, that the foundational issues are currently under renovation, and that the result produces MuseScore 4.1 properly dressed in an gleaming, elegant multi-window environment.