At Hackaday Supercon in 2002, the badge for attendees (https://hackaday.com/2022/10/12/the-2022-supercon-badge-is-a...) implemented a fictional 4-bit CPU along with control panel for directly entering instructions and running and stepping through code. I had a huge amount of fun implementing a space shooter video game on it, as the panel included a bit-by-bit view of one of its pages of memory. Comparing its Voya4 architecture with the 4004 was fascinating. Some similar tradeoffs, but the Voya4 has the benefit of 50 years of CPU instruction set exposure.

Alas, dimitygr's method wouldn't work on the badge, as the memory and RAM are all internal to the PIC24 that implements the CPU emulator.

BTW, 4-bit CPUs are still made and used. Many of the mass-produced IR remotes are programmed using a 4-bit MCU. See https://www.emmicroelectronic.com/sites/default/files/produc... for a datasheet.