shutting down the Mac Mini M4 every day

No harm shutting it down every day.

Also no harm leaving it on 24/7.

It is helpful and recommended to restart the Mac from time to time to allow the OS to do housekeeping - maybe once a week.

I suppose some people leave their house lights on 24/7 as well as their computers. That all seems so unnecessary.

There was a time with the old spinning hard drives that leaving a machine on was desired because of the long times it took to boot when starting up. With the current machines and the high speed SSDs, that is no longer the case.

Personally, I keep my M1 Mac powered 24/7 and simply use Lock Screen. I take advantage of having it run 24/7 by scheduling backup tasks and other activities while I am asleep. Using Lock Screen will eventually kill the video to the display and the display not seeing video for some period of time will put itself to sleep.

However, if you are not comfortable leaving it on all the time, by all means shut in off when not in use. Either way, nothing will get harmed.

If you want to start/shutdown daily I would recommend an app like Onyx that has a utility that lets you set the time for startup and shut down. Then you do have to worry about using the bottom power button to start each day.

Harmless but 100% unnecessary too.

Avoid Sleep - unnecessary and can cause mystery issues. Shut is best.