Have you linked the remote for your Express 4K to your tv?
The power, volume, and mute buttons on wifi-direct Roku remotes send the appropriate signals to your TV via line of sight infrared (IR) signals. Different tvs have different sets of IR codes they accept, so you have to set up the remote to send the correct code set for YOUR tv.
Roku menu path: "Settings -> Remotes & devices > Remotes" At this point if your remote has already been setup for the tv but you need to reset it, arrow down to select your Remote (in use). Now select "Set up remote for TV control" .. And follow the prompts
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Streaming Stick 4K+ 3821 | Streaming Stick+ 3810 | TCL Roku TV 43S245/C107X
I don't have a Roku remote that controls a TV, but I think the way the Roku code selection done is that it keeps trying stuff and asking you if the music has stopped. You can try lying - ie: say it has not, the first time it stops to see if a different code also stops the music. (This is because some devices have multiple codesets in which certain functions overlap.)