I apparently successfully upgraded a custom configuration of Stretch (fully updated) to Buster on an RPi 3B+. It boots and runs OK on the 3B+. It won't boot on the 4B. The green activity LED flashes for a brief time and then nothing happens. There is no output on the display.
Do I have to do something to Buster to make it boot on the 4B? Can the same SD card be made to boot on both?