It works, even when hybrid booting from SD with the OS on a USB 3.0 SSD. There were no official Pi4 power supplies available when I ordered mine, and I already owned an official 2.5A Pi3 PSU and some Volutz micro USB to USB-C adapters, so that's what I'm using now.
Since pure USB boot is not enabled with the current firmware, I have /boot on a micro SD card and / (root filesystem) on a SATA-III SSD with a USB 3.0 to SATA-III adapter cable. Here's a picture of a similar setup on my Pi3. Same configuration for my Pi 4B, and it works fine using the old 2.5A power supply with this adapter. I'll get the official USB-C PSU eventually, but for now this works for me.