I'm pretty sure rpi3 as well.
When I started it worked out of the box on rpi3 already and a year after rpi4 came on the market the firmware was updated to support it there as well. New pis ship with recent Firmware, so they work out of the box, early rpi4 might need flashing.
You'll find plenty of tutorials if you Google "RPI 3/4 USB boot". I run mine from a SSD in a sata-usb adapter. The storage space and peace of mind, not having to fear corruption is definitely worth it (also SSDs are dirt cheap right now (just make sure you have an adapter that supports all block device access modes if you need all the speed you can get, there is one that is not always supported by the adapter)).
(Edit: sorry, only talking about the B+ variant! I don't have experience with other variants)