Sounds like it might be hooked up incorrectly.
On the solenoid, two of the four pins are always hot (they get power from the solenoid wire) and were originally designed to provide outgoing power to the ignition switch and then incoming power from the regulator/rectifier.
The other two pins go to the yellow/red wire and green/red wire, used to activate the solenoid.
With the plug removed from the solenoid and everything hooked up to the battery, use your multimeter to identify which two pins have 12V. The other two are the ones that should go to the yellow/red and green/red wires (doesn't matter which goes to which).
On the solenoid, two of the four pins are always hot (they get power from the solenoid wire) and were originally designed to provide outgoing power to the ignition switch and then incoming power from the regulator/rectifier.
The other two pins go to the yellow/red wire and green/red wire, used to activate the solenoid.
With the plug removed from the solenoid and everything hooked up to the battery, use your multimeter to identify which two pins have 12V. The other two are the ones that should go to the yellow/red and green/red wires (doesn't matter which goes to which).