How to open scores from a newer Musescore 4 version with an older version

The setting described here only works in MuseScore 4.2.1 and later, but not before!

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 恢复出厂设置).
Open it in an editor and add
[project] disableVersionChecking=true
if the section [project] exists already, only add that disableVersionChecking=true to that section.
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.

An alternative might be to use MuseScore 3.7 Evolution, which can open all MuseScore 4 scores. See https://github.com/Jojo-Schmitz/MuseScore/wiki for details.