I have just tried booting my old[ish] 1Gb memoory Pi4 from USB instead of from a micro-SD card. I just copied the SD card to a USB stick, plugged the USB and turned on. It works, no hassle at all.
The 'oddity' is that the green activity LED now flickers regularly once a second, when booting from the SD card it's on for a while to start with, flickers randomly for a while and then is essentially off all the time (with tiny flashes occasionally). Booting from the USB is similar at the start but settles down to quite a long flash of the green light at what looks like one second intervals.
It *feels* like the USB 3 stick is faster than the SD card but I've not really checked carefully to see if this is really try.
While I'm about it - what a load of pointless waffle there is about booting a Pi 4 from USB if you do an internet search. I did a quick look at some of them, pages of instructions which basically boiled down to 'write the Pi image file on a USB stick and plug it in'! It works just the same as an SD image. The only thing that might be of interest is that you can set the order of looking for the image in the firmware so that either SD card or USB is checked first.