By coincidence (searching for 6-bit and paper tape simulators), I found this project, first an FPGA remake
(3 parts post inclusing emulator, but obviously not finished yet)
https://www.designnews.com/electronics/reincarnated-return-hrrg-4-bit-computer-part-1
The original one by Max Maxfield and Joe Farr is here, 4-bit data bus and a 12-bit address bus
https://www.eeweb.com/building-a-4-bit-computer-from-the-ground-up-index-of-articles/
Next to some other 4-bit and 8 bit computers there and here
