Know this, that ivy bridge cpus are hot. They are hotter than sandy bridge chips so keep that in mind when overclocking.
Also know the v6 isn't that great of a cooler...so you need to be carefull and always watch your temps.
However, I believe that as long as you don't go over about 76C you will be perfectly fine. So technically you could push it a bit more. you could probably get it to 4.4 and leave it at that as long as the temps stay safe.
But do you need to go any higher? that is the question. If you really really need that bit of extra performance and speed, then do it...you still have a bit of wiggle room...If you don't really need it...why bother?
Also I suggest you use intel burn test to correctly test the stability and max temps...