Virtual Studio Technology (VST) is an audio plug-in software interface licensed under Steinberg that integrates software synthesizers and effects units into digital audio workstations. Most VST plugins are either instruments (VSTi) or effects (VSTfx); VSTi includes software simulation emulations of well-known hardware synthesizers and samplers.
In MuseScore 4, any compatible VST plugins installed on a Windows or MacOS computer will automatically be made available in the Mixer, where you can easily configure playback settings for all instruments in a score. Note that Linux is not yet supported (but see Linux VST3).
After installing new VSTs on your computer, you may have to restart Musescore 4 to make them visible in the mixer. If this doesn't work, you can force a re-scan of VSTs by deleting previous cache settings before restarting. On Windows 10 the relevant file can be found at
C:\Users\ [your user] \AppData\Local\MuseScore\MuseScore4\known_audio_plugins.json
MuseScore 4 directly supports VST3 plugins only (not VST1 or VST2) because of licensing restrictions. If you are using VST1 or VST2, see How to use older, non-VST3 plugins in MuseScore 4.
Note: Sound settings in the Mixer are saved with the score, but not to the program.
See Mixer: Sounds and Audio FX.
Alternatives to VST instruments: