Until foundation releases a “firmware” update for Pi4 that allows USB booting, you’ll need to put the boot file system on uSD but the root file system can be SSD. (I use a script called ‘usb-boot’ found in a forum that moves the root system from uSD to SSD and fixes all the info in database.). However, there are lots of reports about some USB3 chipsets not working well with Pi4. Google is your friend here for finding a chipset that others have reported as successful.