I wired the electronics and the motors together and give it a shot!
Everything work as expected. Since the "enables" will only disable when they're logically low, and therefore, I could skip them at the time being to make the wires less messy.
So right now, I just need to connect the electronics to that ST-4 socket, and the next step would be to make a ST-4 compatible hand controller to try out motorized hand guiding. If everything work, all I need would be a ST-4 compatible guiding interface (GPUSB, etc) or a guiding camera which could produce ST-4 signals.