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

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)

Since MuseScore 4.2.1, open it in an editor and add

[project]
disableVersionChecking=true

Save, close, restart MuseScore
It'd then stil show the dialog, but hitting OK (the only option) it'd load the score

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