The Coin Unit stepper inside the cabinet conrols the number of players. It has 4 positions. Pressing the plunger into the small coil should reset the stepper to a 1 player game. Pushing the plunger into the large coil will advance the stepper for additional players. You can tell how many players the game is set up by checking the backglass in the Can Play area.
When the 4th player position is reached on the Coin Unit stepper, a peg on the gear opens a switch. If this switch gets opened in a position before the 4th, I believe it can prevent additional players from being added. There is a great photo on pinrepair.com showing a coin unit stepper in the 4th player position with the switch open..here
http://www.pinrepair.com/em/wcoin1.jpg