instantOS arm
For a Raspberry Pi 4, you can mostly follow this Installation Guide.
Todo:
https://github.com/instantOS/instantOS-arm/projects/1
Supported hardware:
https://github.com/instantOS/instantOS-arm/projects/2
Where to download?
http://www.gardling.com/instantos-arm There is both a full and lite version of the tarball. The full version has instantWM with all the instantos programs for a fully functional desktop. The lite version is without any desktop environment, but still has it in the repo.
Other approaches
If you are a fan of the Nix package manager, you may want to try instantNIX with Raspi 4B instructions (not the recommended way).