Total system voltage will of course be lower while cranking. This is due to the draw from the starter. I would think it would be more than 9 V if your batteries are in good shape if you get 9 V on the purple wire at the switch, but only 4 V on the coil positive then your problem for sure is in the purple wire going to the engine connector. The dead man switch is in line with that purple wire going to the engine. It sounds like you may have found the issue. Or at least one of them. Take one lead from your meter and clip it to the red connector on the back of your switch. Take the other lead and clip it to the purple wire connected to the I terminal. When you crank the boat, your meter should read just millivolts at most. Any more would indicate that either the switch is bad or for some reason the I terminal does not get power when cranking.
