I am building the RPi 4 as a desktop for an associate who specifically requested Ubuntu Mate with which he is already familiar.
I have as yet been unable to find a definitive answer to this question in any of ubuntuDOTorg, ubuntu-mateDOTorg, raspberrypiDOTorg, and now here ubuntu-mateDOTcommunity/c/support/raspberry-pi.
It feels a little like "Ubuntu Mate on Raspberry Pi 4" is something of a Twilight-Zone case. I plan to build a second iteration and will use Raspios for comparison, but meanwhile will try to bone up on logfiles to see if I can find clues for where things are done.