I am having the same problem as @Kagekowai describes: ~2 quick flashes of the activity led, then nothing. Same problem without sdcard. As this seems to be a EEPROM problem, I created a sdcard with a FAT partition, copied the contents of the zip with the eeprom recovery stuff on it. But that is handled exactly the same: no continuous activity flashing after 10 seconds or anything. I had the RPI4 connected to my PC at some time, and realized that the PI does OTG on the USB C port, and shows itself as 'BCM2711 Boot', vendor 0a5c, product ID 2711. It apparantly can boot over OTG, like RPI0, RPI0W and the Compute boards. It will apparently soon be supported by usbboot: see https://github.com/raspberrypi/firmware/issues/1184
But alas, my rpi4, that I waited over 6 weeks for, is going back. Hope to get a replacement sooner..