It looks like RPi Foundation is working on a duel boot system that will allow direct boot from USB...  (now BETA).

https://www.youtube.com/watch?v=IorifrSDmQ4  Raspberry Pi 4. USB SSD Boot Official Beta. See tutorial in next video

https://www.youtube.com/watch?v=AskGItLhESk Raspberry Pi 4. Tutorial Official USB SSD Boot. ## at 3.38 extra step see pinned comment ##

OR the "old" way...

https://www.youtube.com/watch?v=5Qgno42VGPY #320 Raspberry Pi 4 with SSD simpler: With Berryboot

But it requires uSD for initial boot.

I have a SSD on order so I can try.  It should also work with a USB3 thumb drive?

73, steve WB3LGC