RPi4 doesn't need bootcode.bin or a SD card, everything's in the boot eeprom - just make sure it's up-to-date and network boot is enabled in BOOT_ORDER.
Copying the contents of the FAT partition on the SD card to a directory served via TFTP and NFS and creating a NFS share for the storage partition is enough - the bootloader in eeprom will fall back to start.elf if start4.elf can't be found.
so long,
Hias