Did you follow the instructions and attempt to reload the EEPROM ?
If your Raspberry Pi 4 will not boot, it is possible that the SPI EEPROM has become corrupted.
To check, remove the SD card, disconnect the device from power, then reconnect it. If the green LED does not flash, you will need to reprogram the EEPROM:
Download the bootloader
Extract it to an empty FAT-formatted SD card and insert it into your Raspberry Pi 4
Connect the power and wait for the green LED to flash quickly