I installed ubuntu Ubuntu 19.10 on raspberry pi 4. it booting and working fine from SD. How to boot it from USB SSD.
As Kurankat has answered, there in no direct SSD boot yet.
Nevertheless, it is possible to run the system from a USB SSD after the initial boot from the SD. To put it very simply, you flash the system to both an SD and the SSD (or flash to an SD and copy to an SSD), and then "point" the system to the SSD using the cmdline.txt file.
So after the boot from the SD, everything runs from the SSD. This is fully described at:
There are two issues to be careful about:
The "quirks" issue described in the link above, relating to SATA-USB adatpters (if your SSD needs one)
For Ubuntu, cmdline.txt is not in /boot. Instead, you have to edit nobtcmd.txt in /boot/firmware.
According to the Raspberry Pi 4 documentation:
Support for these additional bootmodes will be added in the future via optional bootloader updates. The current schedule is to release network boot first, then USB boot.
So I don't think you can boot from the USB yet.