You got a very much OCable CPU and an excellent air cooler, kudos to the choices.
You should get to 4.5 GHz if you don't have a 'VCore tantrum' or 'overheat tantrum' CPU, which hopefully you won't have.
I won't recommend sustaining anything above 4.6 GHz. Try to remain in the early 70s temp bracket and most importantly remain stable.
Its a trial and error, look where you can go without tampering with VCore, hopefully you can get to 4.5GHz without problems.
Intel has affinity with certain voltage, so don't tamper with it too much. Avoid going above 4.7 GHz.
My safe zone ends at 4.5 GHz, though 4.7 GHz should be achievable, but don't sustain it.