@Ed, yes I agree about bulb brightness. I had originally asked TJ to buy 2 bulbs so that he could compare L/R. Although comparing both sides would only assess the two separate power feeds, since the grounds are partially shared - high resistance ground could be a problem, but then he is also doing multimeter measurements,The only problem I see, which you alluded to about bulb brightness, is that 8V will likely still power the halogen bulb. It will obviously be lit at a dimmer level than it should be. Whether that difference is visible may depend on when/where it is checked. It also could be that the HID ballast won't run on 8v, but is otherwise working fine. I was/am assuming the HID system was out of the circuit once he unplugged the H4 to check voltages.
With those measurements, I’d say you have a broken R/Y wire on the first 3 that fixed itself on the last measurement. Alternatively, you could have made Measurements with the headlights turned off for the first 3 and then turned them in for the last one.Batt (-) to R/Y (low beams): 17.0 mV
Batt (+) to R/Y (low beams): 12.25 V
Batt (-) to R/Y (high beams): 0.746 V
Batt (+) to R/Y (high beams): 0.669 V
I hope you’re starting to understand what you’re trying to do with the multimeter. You should also consider buying yourself a normal test light or consider getting a power probe. Google “test light” and “power probe” On YouTube sometime and you’ll see how those tools will make automotive electrical diagnosis a lot faster and less confusing than a multimeter. You might also consider looking up “dual impedance multimeter” and how that can get rid of the false multimeter readings such as ghost voltages.