I had a Fan Shim on my Pi 4. I initially setup the Daemon to turn the fan on off based on temperature. Eventually I just ran it without any of the software and let the fan run continuous. My temps are a pretty constant around 40c. The LED I don’t use, I had a BLINKT showing temps. I eventually switched that to show CPU load. The Button by default will boot the Pi up if it was shut down and left powered. It grounds GPIO 3 when pressed. I also did a config.txt edit to have it also do a proper shutdown when pressed.
Later on I added a pull down resistor to GPIO 18 on the fan shim to turn the fan off by default. If I do a shutdown with the button the fan now turns off. without the pull down resistor it just keeps running until you unplug the power supply. Not what i wanted. The downside is it now has to be commanded on. That I did with another config.txt edit.
It turns on at 35c now.
EDIT: I said “had” because that Pi 4 in now in a SmartiPi V2 case, that comes with a cooling fan. I found it a bit on the noisy side and connected it to 3.3V instead of 5V. I hardly notice it now and my temps are just a tad over 40c. I am considering switching the stock fan out for the fan shim though.