I would say that hitting 4ghz on air is going to be a challenge, not impossible, but a challenge nonetheless. Chances are about 15% I'd say - from what I have read of other peoples experiences, about 3.4-3.8 is the norm.
Go Watercooling and you'll hit it, and more, no probs (well, more chance of it happening anyway)
I have got the Noctua cooler (suggested above by Reflux) and a Antec 1200 (great air flow) and I can only achieve 3.8ghz stable (which is still an approx 50% o/c and not to be sniffed at).
When you get your kit, don't get downhearted if you can't get 4ghz on air. You will either have to be extremely lucky or live in a house with broken central heating.