This new (as of June 2019) text has just appeared on the Raspberry Pi official Downloads Page:
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
One important consequence of this is that my oft-given assurance that an RPi board has no user-modifiable persistent memory and is thus always 'factory fresh' at every Power On is henceforth not accurate.