cartft.com published some info on how G4 steering wheel interface works and a software utility to tune the G4 interface to your particular car. It's supposed to work with any resistive-ladder type controls - the software prompts you to press the steering wheel buttons one by one, measures the resistance and stores it for future matching. Most of the program interface seems to be in Korean, but it's pretty intuitive
Seems to work ok with InfillStation software, as far as I can see touching the wires with different resistors while G4 is still sitting on my desk at home
The hardware part (ADC?) seems to be connected via USB, because when you run the utility on a normal PC (not G4) it complains in Korean but the letters USB are clearly present in the message
http://www.cartft.com/support/drivers/Infill-G4