A voltage divider with those resistors at that voltage should have just under 4.5 volts. If you're using that voltage to logic check or reference something you should use a zener or internal PIC voltage reference as a standard. There is some variation in voltage with engine running and off. Why not use some of the 5 volts from the power zener and voltage divide that to get around 4.5 volts? More stable and accurate. Though I still don't see how a voltage divider replaces an optocoupler.