Thanks for sharing. I've just managed to boot ubuntu mate (arm64) on rpi4 using the second method. I used the /boot partition of "official ubuntu server for rpi4 (arm64)" system and /root partition of " ubuntu mate for rpi3b (amd64) " system. Despite there are blutooth and display driver issue on official ubuntu server system, my "assembled" ubuntu mate seems to work all fine. There IS a small problem that ubuntu mate on rpi4 seems to regard /boot and /root as removeable drive (like a usb storege), not a local disk. But that doesn't matters much.
Now that there is an official Ubuntu Mate 20.10 support for the pi4, is there any way to do this without a pi3? I don't have one, and am looking for 18.04 specifically.
If anyone knows, thanks in advance.