I would say 4.5 ghz is the maximum (for that cooler) as well given you have real nice airflow and you have a second fan on the heatsink which does make a difference of 2-5 degrees which means a lot to me when overclocking. Anyways lowering voltages also helps bring temperatures down, so my advice is try to get 4.5 ghz with the lowest possible voltage that still is stable. I know for a fact you can reach 4.2-4.3 ghz with that cooler easily compared to 4.5 ghz.