The Raspberry PI Foundation has released beta firmware and a bootloader to enable booting the Raspberry Pi 4 from a USB mass storage device.
The new firmware and bootloader could be useful to Raspberry Pi 4 owners who want the option of booting from a faster USB mass storage device instead of only doing so using an SD Card. This is a beta release. If you aren’t already comfortable with manual firmware updates then please wait until this is available in a standard release. It won’t be very long.
Link: New Raspberry Pi 4 update beta lets you boot from a USB mass storage device