Hi,
You can't really measure the IVR output voltage with a conventional voltmeter. The original IVR is a thermal/mechanical device that slowly switches between open and +12 volts. On a conventional meter you would see this pulsing, not a steady voltage. I'm puzzled if you are indeed seeing a steady 4 volts. The gauges are thermal devices that average this pulsing voltage into a heating value of something around the equivalent heat value of a steady 5 volts.
If you are seeing a steady voltage , either someone has already done an electronic conversion or something is messed up somewhere.
