The My4TH XS is the epitome of minimalist computing, boasting an incredibly compact design with only 13.75 ICs. By reducing the microcode counter width and eliminating unnecessary components, the My4TH XS is extremely small but slightly slower than the original My4TH. The My4TH light is a step up, featuring an I2C interface for faster connections and a larger ROM with additional functions like floating point support. Both boards run on a Forth 2012 operating system and offer unique opportunities for learning and experimentation. However, note that the My4TH XS lacks non-volatile memory and I/O, and the My4TH light shares a digital input for serial and I2C communication.
http://mynor.org/my4th_xs.htm