I have tried both Arm7 and the ArchLinuxARM-rpi-4 and the ArchLinuxARM-rpi-aarch64 Arm8 installs on an RPI 400 all in one.
The Arm7 install never shows anything on the screen, just repeatedly power cycles. The Aarch64 flashed the message
$this->bbcode_second_pass_code('', 'Bus xhci_probe failed error -110 no working controllers')
on the screen in a loop. I managed to boot Aarch64 by replacing the kernel with the kitchen sink version from
https://github.com/raspberrypi/firmware but of course all /lib/modules don't work among other things and its not a viable way forward.
Are others having success with RPI 400?
I have a PI-Zero running ArchLinux and its super stable.