The firmware included with the Raspberry Pi 2 Model B version 1.2 introduced the possibility of booting from a USB device: before that, it was only possible to boot from an SD or microSD card. But ...
If you are considering booting your new Raspberry Pi 4 from a solid state drive (SSD) you might be interested in performance testing carried out by Avram Piltch over at Toms Hardware. Providing ...