Is it possible to have DietPi root on usb drive for Odroid C4?

AFAIK, it is sadly not possible. On Odroid HC4 it is theoretically possible (coincidentally I’m just confronted with it: Odroid HC4 booting from SSD) since it has an SPI flash it can boot from. The bootloader from the SPI flash can then load an OS from USB. But the C4 has no SPI flash and I doubt that the hardware/firmware itself can check USB devices for a bootloader.