These are the instructions to setup a custom Ubuntu 18.04 Bionic on Raspberry Pi 4 B.
Obtain Ubuntu 18.04 ARM Image
To install the long term supported (LTS) Ubuntu 18.04 on the Raspberry Pi 4 B James A. Chambers provides a blog post where he created a custom image. His unofficial releases can be found in his github repository.
After flashing the image to the sd card insert it to the Pi, hook it up to a monitor via HDMI and power it up.
Once logged in (default password is
ubuntu which needs to be changed after the first login) update Ubuntu:
sudo apt update sudo apt upgrade
Reboot and follow the next steps to install ROS Melodic.