My i7 920 idles at 50ºc and under about 1 minute of prime95, hits 95-100ºc. it's OC to 3.8 and i'm using the dark knight hsf. core voltage is 1.336 if that helps.
Downclock and reduce voltage or reseat the heatsink.I'm not sure if the xigmatek dark knight can cool an i7 920 at 4ghz but you have to keep the temperature below 80c and as some prefer maybe below 75-70c.
Sounds like you might have cooler issues. My 920 is cooled by a NOCTUA NH-D14. O.C. to 4ghz (1.25v). Idle 43-46 and after 20 passes of Intel Burn max temp 69-71. After 24 hrs Prime 95 max temps 68-71. Please note the the room ambient temperature has a lot to do with your temps. These are not exceptional values and I hope they give you a comparison.
Ive used that cooler on an i7 before and its a no go for that high an overclock.
I got fed up and bought a H50.
That cooler is weak for i7. I7 runs hot especially when overclocked, you need real good cooling.
I'm currently using The Xigmatek DK on my i7 920 clocked @ 4.0 GHz and haven't had a problem, temps don't get above 75c.
OP: Maybe you should re-seat your heatsink as others have posted. I had to do this because I wasn't happy with my temps the first go-around. Just apply a small amount of rubbing alchol onto a Q-tip, then slighty rub that over the old thermal paste on your HSF and CPU. Then get everything clean and apply more thermal compound and attach your cooler and you should see better temps.
Another thing I want to mention: Ambient temperatures, if the location to were your PC is at is quite warm, it will have a substantial effect on your overall temperatures.