Four NES controllers to USB using Arduino Leonardo board
Clone the project locally, then in a shell terminal type the following commands
git submodule init
git submodule update
make
To flash the Arduino Leonardo, plug it and push the reset button, then type
make avrdude
Note: If you are on Windows or if you already have other devices named "/dev/ttyACMx", you need to edit the makefile and change the "AVRDUDE_PORT" variable so it matches the right device.
I don't know if it is a design issue but it seems like flashing the Arduino Leonardo can be a bit tricky sometimes and requires some timing. So, if you have any errors during the flashing process, unplug/replug the board, try again or look on the internet for additional information.
This Fritzing diagram shows you how to connect the four NES female ports to the arduino leonardo.