i have a hyper 212 evo but what i'm wondering is what voltage i should set for a 4.2ghz overclock
Nobody can tell you what´s going to work with your cpu, you can go that far stock but others can´t do that, you just have to see it for yourself.
Up the multiplier 1x or 0.5x if you´re past 3.9ghz, shut down all the features like turbo, cool n quiet and stuff, test the cpu with prime 95 or intel burning test until you find that clock go bsod, then up the voltages little by little. Most Piledriver CPUs will run an extra 400-500Mhz on Stock Voltages.
For more info check this link:http://www.overclock.net/t/1348623/amd-bulldozer-and-piledriver-overclocking-guide-asus-motherboard