If you're skeptical that this feat is possible with a raw 4004, you're right: The 4004 itself is far too limited to run Linux directly. Instead, Grinberg created a solution that is equally impressive: an emulator that runs on the 4004 and emulates a MIPS R3000 processor—the architecture used in the DECstation 2100 workstation that Linux was originally ported to. This emulator, along with minimal hardware emulation, allows a stripped-down Debian Linux to boot to a command prompt.

that is 2^8 levels of insane! and of course its Debian.

edit: 4bit data 12bit addressing make it an 8bit processor ; -)

I will slowly corrode on this hill.