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Jump 12v to the wires on the clutch safety switch (gear selector sw if auto). One of the wires on the clutch safety switch goes to the starter solenoid terminal. The starter should crank. If not, jump 12 volts to the starter solenoid terminal on the starter and the starter should crank.
You can do a quick check of the starter, by jumping from the large, battery cable connection on the starter to the small "start" terminal on the starter. Often, you can do this with a screwdriver. Be prepaired, for action, however, because it is likely to crank. I assume the 4 volts at the small "start" terminal at the starter, is with the key in the "crank" position. In anyother position, that terminal should be dead. If this is the case, you are close to tracking down the problem. However, it could be either before or after the ignition switch. In fact, that would be a good place to start. Check the voltage feeding that circuit on the switch, and the output at the switch, to the starter. That will tell you if the problem is before or after the switch. It sounds like you're on the right track. Let us know what you find. Good luck, and ...