That diagram looks to me as being wired in series. Eithier way I would do a relay for each light max power available for them no matter what and if one fails you don't lose bothights. Redundancy is a good thing when it comes to safety equipment. I will admit I am not the best at reading wiring diagrams but I am learning. If I read it wrong please explain where I went wrong so I will know for next time
The red wire goes to both headlights and branches off. Parallel.
If it were in series, the red would go to one light,the negative for that light would go to the positive of the second light, then the negative of THAT light would go to ground.
And two relays to troubleshoot are much better than four. This "starving of lights" **** needs to stop. Spreading misinformation based off of your lack of understanding of electrical systems. If a pair of lights is running in parallel, they're both getting full voltage, as long as the relay can handle it. Two HID ballasts are 6-10 amps each on startup. Most relays are 30-40 amps. Enough to run three or four HID ballasts on one prong. I wouldn't worry about a relay failing from high amperage.
Another thing to think about with running high and low off of separate relays: Let's say your high beam relay "sticks" on. You now have juice going to both your low and high beam wires in your lights. If your two-relay system relays fail, either your lights stay on full time (then you replace your SPST relay), or your high beams stay on (then you change your SPDT relay). You're not sending juice through every wire. Having two relays instead of four (you still need the SPDT relay to switch between your two dual-output SPST relays, and the first SPST on/off relay to power the whole spiel) is just a waste of time, energy, space, and money.
Not to mention the limited sources for those KC style dual-output relays. As opposed to 2 bucks a pop for the standard SPST and SPDT relays at some vendors, you're paying $6+ per KC relay. And you're going to want spares.