Basically the gist is this:
If you are not pushing any buttons, but the Player 3 / Player 4 LED's are flashing a button code, you have a bad, shorted, or missing connection.
If you are pushing a button, but you are seeing two or more (multiple) codes flashing on the Player 3 / Player 4 LED, then you have a short-circuit between two buttons.
When you are running the button test code be sure to hold down the button to get a correct reading, you want to press just one button at a time and see a single LED code flash over and over again.