Historically, booting a Raspberry Pi required an SD card. However, if you follow [tynick’s] instructions, you can get a Pi 4 to boot from the USB port. Combine it with a small solid state disk drive, and you’ll get great performance, according to his post.
The caveat is this depends on a beta bootloader and, of course, you’ll still have to boot from an SD card at least once to load that bootloader. If you were deploying something serious, you’d probably want to make sure the bootloader is suitable for your needs.