As your parent says, the Pi 4 doesn't support USB or network booting yet. From your link:
> Support for these additional bootmodes will be added in the future via optional bootloader updates
I believe they need to write (bootloader) drivers for the PCIe host and for the new USB3 chip before USB boot can be supported by the bootloader, while netboot requires a driver for the new gigabit ethernet core.
> Support for these additional bootmodes will be added in the future via optional bootloader updates
I believe they need to write (bootloader) drivers for the PCIe host and for the new USB3 chip before USB boot can be supported by the bootloader, while netboot requires a driver for the new gigabit ethernet core.