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.
In fact, most of the work here is doing the prep work. You have to grab the new bootloader and flash it. You also have to make some changes to get the boot code to check the USB port. You also install the operating system image on the SSD just like you would an SD card.
After that, it is easy to copy the boot files to the USB drive and you are ready to go. This would be a great use for an old SSD if you pair it with a USB adapter.
According to the post, the SSD is about ten times faster than the SD card, so the performance boost is significant. Of course, some of this depends on the USB adapter, and [tynick] recommends a particular one that seems to work well.