With air cooled you can get up to 4.2ghz. Especially with that cooler. Also, when overclocking as I dont know if you're new to it or not. Dont just get and it and then OC it to 4.2Ghz. Slowly start working your way towards it. And stress test for about 15-30 mins. Some people will say longer but I do it for about 4-5 passes in prime and go higher.
I'm not new to overclocking but thanks anyway. So btw is the motherboard and cooler enough yes?