Going for a 4 GHz overclock I would look towards a Prolimatech Megahalems Rev.B CPU Cooler
or the slightly better but more expensive Noctua NH-D14 120mm & 140mm SSO CPU Cooler
I am currently running a PC with ASUS P6T Deluxe v2, i7 920 and a Prolimatech Megahalems and getting to 4 GHz was no problem at all, however this will entirely be down to your CPU, all are unique.
Temps are great at that speed, mid to high 60s running Prime95 small FFT test.
If however you find both of these too expensive, then a cheaper option would be the Scythe SCMG-2100 Sleeve CPU Cooler
, though cooling is not quite as good as the above mentioned.