Ghostnotes?

Select those notes and reduce their velocity from the inspector.

See the example here:
https://musescore.org/en/node/298624#comment-966654

I just downloaded Musescore 4 and am using it to transcribe drum parts. I want to include ghost notes. I saw a tutorial online but it was using Musescore 3.something and directed us to Inspector where it was possible to alter individual notes.

For new people with Musescore 4:

Under Palettes you'll find the Grace Notes Menu where you can select from a number of options. If you don't see Grace Notes, go to the View menu and select Master Palettes. You can see all palettes there and add the ones you want to your dropdown menu along the left of the screen.

To adjust volume, you now go to the menu on the top left called "Properties." Under Properties you will see a box called Playback. Click on that and a Velocity box will appear where you can adjust the volume of individual notes. Be aware, though, that you must have some kind of score created and you must have selected an individual note. If you are new to Musescore and are just investigating the system, you will not be able to see Playback and Velocity without having clicked on an individual note in your score.

Another way to create ghost notes is to subdivide the previous note and use that note to create a ghost note. So, you would, say, take your 1/8 note hi hat note on the one-and and make it a 1/16 note. Keep the hi hat note and add a snare to the new 1/16 note. De-couple the bars joining them using the beam palette. Then go to Properties and Playback and Velocity to adjust the new snare note. Depending on the song you are creating or transcribing, you can play around with the various ghost note notations and sounds to get the Playback to be as you want it.