The S1 project provides extensive documentation on the inner workings of the Native Instruments Traktor Kontrol S4 in addition to an open-source DIY kit that upcycles (opens new window) the no longer supported unit.
Not only keeping it from the landfill but also transforming it into a better more versatile device.
The project documents everything about the S4's internals on it's Github page (opens new window). After reverse-engineering the original motherboard new aftermarket boards were created to enable the new units to function completely independently in a class-compliant way to ensure future compatibility.
A custom and compatible FX board had to be created since the Kontrol S4 uses one long horizontally placed daughter board for its entire FX/gain section.
This board daisy-chains into the S4's native serial communication which is used to control all the LED's and read out all the buttons.
One can manufacture the boards using the files provided in the Github repo or buy all the necessary parts as a kit.