The Coin unit has a last position break switch that should open after you've added the 4th player but is probably installed wrong or out of adjustment. It has a White-Brown wire on one terminal and a White-Blue wire on the other terminal.

For a switch to work 3 things are necessary:
1) When open, there should be a small space between the contact points (duh)
2) When closing, the long blade's contact point should push the short blade's contact point enough to move the short blade
3) The contact points should be clean, which they usually will be if #2 is happening