Well i got the E8400 running @ 4ghz. I can squeeze 4.3 but then I care less about the 0.3 and I find that after 4ghz it gets a lot tougher.
Bump your voltage to like 1.35V.
Well, on my E8400 I have Bus speed @ 450 and multiplier on 9 (4050mhz)
So for you, maybe keep the multiplier on 9 and then increase bus speed little by little? Start with my settings and see how it goes. Then from then on go up little by little (if 450 is good, try 470, and then 480, and so on)