If you wish to change the sound of an instrument using the Mixer as was possible in MuseScore 3, see: https://community.masteringmusescore.com/c/resources/ms-basic-individua…
Using the file available there you can select individual sounds (via the Mixer). You do not have to change the instrument in the (MU 4) score itself .
Comments
You might spend a little time in the manual.
https://musescore.org/en/handbook/4/mixer#mixer-controls
If you wish to change the sound of an instrument using the Mixer as was possible in MuseScore 3, see:
https://community.masteringmusescore.com/c/resources/ms-basic-individua…
Using the file available there you can select individual sounds (via the Mixer). You do not have to change the instrument in the (MU 4) score itself .