His Nibbler implements a 4-bit CPU using 7400 logic chips. Because he’s come up with the architecture himself he’s taking a lot of steps to check all of his work before committing to a PCB.

Enter the DDL-4-CPU, courtesy of [Dave’s Dev Lab]. The DDL-4 is a project to build a modular 4-bit CPU using bitslice methods. This is where computations are broken down into simple operations ...

Wish your PC booted up faster ... a wonder that it can be done at all – remember that the Intel 4004 was a 4-bit processor ...

An earlier logic chip used as a building block for CPUs. Bit slice processors used arithmetic logic units (ALUs) that typically came in 4-bit increments, although 1- and 2-bit devices were also made.