@Hal58 wrote:
After you prepare the Ubuntu-MATE 18.04 SD card and copy; the saved /boot and /rootfs/lib/modules data, you need to edit two files. The first is /boot/cmdline.txt. Since the PARTUUID value for this is for the Buster image, you will need to change this to the PARTUUID of the Ubuntu-MATE image, and likewise, the values in the MATE /etc/fstab will also have values for Buster. The easiest way to do this is to work on another computer (where you presumably did the saving of the buster data). Do a 'ls -lF /dev/disk/by-partuuid' and write down the values for mmcblk0p1 and mmcblk0p2 (if you used a microSD to SD adapter) or /dev/sdx1 and /dev/sdx2 (if you used a microSD card to USB adapter). Now edit the 'partuuid' values into the files. You also need to edit /boot/config.txt file to get the graphical desktop to work. Comment out the line near the end by adding a '#' in front of 'dtoverlay=vc4-fkms-v3d'. Now you should be able to try it in the Pi4 and get some results. Good luck and keep us informed.