To flash the card, I still rely on good old dd on my Mac, but you can use the Raspberry Pi Imager instead. In the future, you'll be able to download it from the regular Pi OS download pagebut for now it's available from this forum thread. I'll be posting a video and blog post with more detail on that experience very soon. And after testing it a bit, I decided to use the Pi 4 as my full-time workstation for a day, to see whether it can cope and where it falls short. So when the USB boot beta was announced, I wanted to put it through its paces. And on older Pis, with the maximum external disk speed limited especially by the USB 2. All you need to do now is run sudo apt-get dist-upgrade -ythen reboot, then your firmware should be up to date.
September Update : USB boot is out of beta! Check out this video for simplified instructions.