The 8700k runs at a 4.3GHz all core TurboBoost by default, so 4.0GHz isn't really an overclock. A Hyper 212 EVO will handle that default Turbo, and you might be able to do 4.7GHz on all cores if you have a decent chip that can do that at less than 1.25 Vcore, but you probably can't go much further than that without a better cooler.