I'm on air with a CM Hyper 212. I haven't experimented too much but I'm currently stable at 4.2, having only changed the base clock to 210 and the multiplier at 20. I did try pushing both (i.e. 220 and 20/210 and 21/200 and 22) without changing the Vcore. I really need to figure out how to increase that! :)
It has to be noted that I'm using a Crosshair V Formula with voltages set to auto adjust, I am not very experienced with this.