Mac MiniM4 with a 1Mac Late 2015
can I reinstall MacOS 10 Catalina after upgrading to macOS Monterey on a iMac Late 2015 so that I can use this as a monitor with a Mac mini M4
iMac 21.5″, macOS 12.7
You cannot use the old iMac as a monitor for the new Mac mini.
Apple deprecated the feature starting with the release of the Retina iMacs in 2014 and killed the feature in software when they released Big Sur.
There are software such as Luna that claim to allow it, but the performance isn't for everyday Mac use.
It would be much easier and wiser to simply buy a true monitor to go with your mini. You can have a screen that compares favorably to the old iMac for ~$300-$400. Less if you aren't too picky.
