Myggan, does the voltage vary between 0 and 4 volts, or is it a steady reading, just different each time you read it. Your description almost sounds like you are measuring the output of the instrument cluster voltage regulator.
If I'm reading the drawing correctly, you should find the instrument panel lights voltage (which would be varied from 12 volts down by turning the light switch knob) at the "inst" terminal on the light switch, at both ends of the fuse that connects to the tan and orange wires and at terminal C on that eight pin connector.