Older versions of MuseScore 4 can't open scores from newer versions (of MuseScore 4)

This works in MuseScore 4.2.1 and later, but not before!

1. Locate the MuseScore 4 settings file (on Windows it is in C:\Users\[USERNAME]\AppData\Roaming\MuseScore\MuseScore4.ini, on macOS it is ~/Library/Preferences/org.musescore.MuseScore4.plist, on Linux it is ${XDG_CONFIG_HOME:-~/.config}/MuseScore/MuseScore4.ini, see Revert to factory settings)

2. Open it in an editor and add
   
[project]
disableVersionChecking=true

   if the section [project]exists already, only add thatdisableVersionChecking=true` to that section

3. Save, close that file, restart MuseScore

MuseScore will then still show the dialog complaining about the version, but hitting OK (the only option), it'd load the score.

Be aware that this is not lossless, features from the newer version will/may get lost in the process.

Unfortutately there's no UI for this, only this manual editing of the settings file