I tried setting up the Rpi 4 to boot from a USB3 SSD using the new rpi 4 experimental eeprom update. I was successful in getting the system up and running. It does work.
However the very first time pacman -Syu is performed the system no longer boots from USB.
EDIT:
Originally I thought that alarm may have flashed the eeprom again, but as moonman has pointed out alarm does not flash the eeprom. It does seem however that alarm comes with it's own version of the *elf and *.dat driver files that it does tend to update.
WORKAROUND:
When pacman overwrites the *.elf and *.dat files in /boot and the USB Boot is broken; To restore USB Boot overwrite the *.elf and *.dat again in the /boot directory as per initial setup.