To the default Ρ for Raspberry Pi 4 boards the ability to boot from USB-drives. Previously, Raspberry Pi 4 boards could only boot from an SD card or over the network. Experimental USB boot support has been in May, but it was not available in the default firmware.
The lack of the initial ability to boot via USB and the long implementation process (more than a year since the board went on sale) is due to a significant reworking of the boot organization in Raspberry Pi 4 and the implementation of USB through a separate VLI controller with its own EEPROM connected via the PCI Express bus.
Source: opennet.ru