The fates disk image is built from the stock raspbian Buster release (which supports pi4), while the norns images are built from monomes own custom linux branch. That Monome branch is meant for maximum compatibility with the norns hardware first and then the new shield.
Pi4 requires a different set of compile instructions than pi3 due to the different processors used.
If I had more time available I’d try to recompile the various linux sources for pi4 + shield, but life is a little too busy for that right now.