If you have 4 volts between the negative post of the battery and ground, then you have a grounding issue. Make sure you clean the battery posts and the connectors and do the same where the battery's ground cable attaches to the frame. You should have ZERO volts between ground and the negative terminal of the battery. Also, there should be a ground wire between the frame and the engine. If that's missing, go buy one and install it. That ground wire should be a braided wire mounted to the frame and a bolt on the starter or the flywheel.
