Many users don't bother to look up Intel's specifications, so some are not getting "over 4 safely". Regardless, D0 Stepping will typically run ~ 200 Mhz faster than C0 at the same Vcore and temperatures.
Intel has from my experience always set their Vcore max very low. I guess just to be absolutely sure that noone can fry their CPU as long as they stay within that limit.
That doesn't mean that running 1.424v would fry your cpu for sure though.
What you suggested yourself is the way to do it. If your cpu runs fine now try and lower the voltage bit by bit and run some benchmarks. When you get a failure you go back up one increment and then run some hourlong tests to make sure it's stable.
Failure from too low voltage doesnt fry your cpu. It just means that there isn't enough juice to do the calculations which doesn't harm the cpu.