You'll need to look at the circuit that sends power to the ball count step up solenoid... Since the machine only has problems during a 4 player game, I'd take a look at the coin unit and the player up unit again. Make sure both of these step up units rotate properly and "click" into each position smoothly when you manually operate them. Check if the coin unit is fully moving into the 4th 4 player position...

If you think the player up unit wires were attached incorrectly, you can use the schematic to see what color wires run between the player up unit and the coin unit.