Hey, I've been trying to overclock my i5-4670k since yesterday when I received my Noctua NH-U14s cooler.
I can only run stable at 4.1ghz with a vcore of just under 1.2v. If I go up to 4.2ghz the Automatic vcore voltage shoots up to 1.397v. And If I try anything about 4.2ghz it just won't run at all. I have been able to run at 4.2ghz using a vcore of 1.275v but that is the lowest I can do.
I just feel like 4.1ghz is not good enough for Haswell and this cooler considering people can run 4.4ghz very smooth. And they can run atleast 4.2ghz without bsod (I got one while playing League of Legends, even though the stress tests ran fine).
Some chips are better than others unfortunately, and as long as your temps on stress test are under ~80 C you should be safe there.
I have a 4670k aswell, and I am running it at 4.5GHz (smoothly as of 3 days). I have a vcore of 1.28, vcore cache of 1.22, and an input voltage of 1.9. What I originally did was use the auto overclocker before looking more into it. I had gotten about 5 blue screens in a months time, and was done with that. It was set set to 4.4GHz, so what I did was used a guide and tweaked those numbers a bit.