In Musescore 3 it seemed like fermatas in one staff sustain sound in the whole system. So the cello tied notes were 'disconnected sounding' because the fermatas from the violin staff caused the system measure to be longer, thus creating a time gap unaccounted for in the cello staff, resulting in cello pausing during the violin fermats. My violin staff has fermatas while my cello staff didn't. Regarding the slur issues, I figured out a problem that wasn't there in Musescore 3. These markings aren't made up by MuseScore, of course, so it doesn't have to be a MuseScore-specific video to hear what 'col legno' means on violin (for example). Videos showing different instrument techniques definitely exist, just search YouTube. I recommend suggesting it on GitHub, which is where issues for MsueScore 4 are being tracked (see the beta announcement for the link). The idea of the tooltips for text markings containing more explanation is good, maybe someday that will be considered. It's true copy & paste doesn't work between versions, but you can open MsueScore 3 scores directly. Not sure what you mean about legato and cello chords - do you mean triplet stops on cello? I wouldn't expect it's physically possible to play those legato! On average, most mightily builds are better than the beta builds though. Occasionally some bug fix introduces a new bug, though, so it can be a little hit and miss. But, in general, nightly builds are more recent than the beta and thus have more bug fixes applied. All builds are just experimental and meant for testing only.

It's important to keep in mind, MsueScore 4 is not released.

Musescore inspector