Not true! The Pi supports several different USB and network boot modes.
https://www.raspberrypi.org/documentation/hardware/raspberry...
This documentation doesn't cover the Pi 4, but USB/network boot is available there too, cf.
https://www.raspberrypi.org/documentation/hardware/raspberry...
> 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.
https://jamesachambers.com/raspberry-pi-4-usb-boot-config-gu...
No, they're not. Network booting is a couple of weeks out and USB over a month. It's a topic of discussion on the Pi forums.