The RPi4 can boot from USB if the RPi firmware is up to date.
Check if you need to update your firmware, I had to.
From: Axel Braun <email@example.com> Sent: Friday, May 28, 2021 12:00 PM To: Mailinglist ARM <firstname.lastname@example.org> Subject: Boot from USB
I tried booting a Raspi 4 with a Leap 15.3 USB stick following the
instructions on https://en.opensuse.org/HCL:AArch64_EFI
which unfortunately did not work.
Any other resources to look for in order to get it running?