Pi 4 arrived on Monday, inserted an SD card that I’d prepared earlier (cloned from Pi 3 and upgraded to Buster) and the Pi 4 booted first time. It was running great for a couple of days, but got home today to it being unresponsive.
Cycled power, still no working system. Removed the SD card and put it back in the Pi 3 and it booted fine.
Put the SD card in the PI 4, no boot. Unfortunately I don’t yet have a micro HDMI adapter to see what’s happening. I’m using an official Pi 4 power supply, but also tried another USB-C power adapter.