Enjoy Ubuntu MATE on your Raspberry Pi 4 Thanks for reading.
Installing the MATE Desktop
On initial boot, you will be required to log in using the username ubuntu, and password also ubuntu. It is recommended to change this password later. Keep in mind that this is the ubuntu-server installation, so you will only have a terminal user interface, meaning at this point there is no desktop installed into the system.
Now to install the MATE desktop environment in the following command in the terminal:
$ sudo apt install ubuntu-mate-desktop
$ sudo reboot