I got 3.9Ghz using the stock cooler and stock thermal paste with stock voltage.
I tried raising voltage on the CPU and I tried up to 1.3v but couldn't get 4Ghz to work. The temps never went passed 70C so they weren't the issue. I put Liquid Metal Pro on it and changed the thermal paste which brought the temps down but still can't do 4Ghz. Once it hits 45C during the stress test it blue screens. So I have given up on it.
I saw some get 4.2Ghz, and I saw a few push 1.5v on their chip, and two killed it that way.