Overclocking is highly variable. There is no certainty your chip will reach 4GHz, or if it does, how much voltage will be needed for it to be stable (higher voltage = more heat). You can find cooler reviews at www.frostytech.com. HardwareSecrets also tests coolers. In general, with a decent fan on it, one 120mm direct-touch tower cooler will be within a few degrees of another. I would suggest one from Xigmatek, Corsair, or Noctua.