There is an easy test for the 4th gear advance system. Start the engine and place the gearbox in the fourth gear position. If the engine revolutions increase about 100 RPM ,then the system is working properly. If not ,then check the electrical first. With the engine not running select fourth gear (with the ignition switch on ) and listen for the click from the solenoid. If no click then check the power to the solenoid. The switch on the gearbox rarely fails. Solenoids are almost as reliable. I say that because when I find them disconnected or improperly plumbed , then most of the time the system works when reconnected properly. I emphasize 'connected properly' because the system will not function properly if the vacuum lines are reversed. One vacuum line comes from the intake manifold to the solenoid and then the other line hooks to the vacuum advance. If you connect it the easiest way then it is wrong. The solenoid is also a dump valve. If the lines are reversed , then the ignition will stay at full advance until the solenoid bleeds down. This system was designed to meet a Federal specification. The thinking was that as you are accelerating through the gears , then the engine is under a load. But when in to gear you are cruising. The system will help on gas mileage and if you have to accelerate hard in top gear , then the manifold vacuum will decrease and there should be little chance of detonation.
Alan T