For the price and if it will fit in your case, the H100 refurb is unbeatable. A few have had leaks but the vast majority have been fine. Just remember, if it goes wrong and you need to RMA it, you need to send it to the Netherlands which could be up to half the price of what it cost in the first place. This put's me right off buying anything that Corsair sells.
Settings for 4Ghz
BCLK 200x20
Vcore 1.28750v (might need more as i had a good cpu)
IMC 1.16250v
PLL 1.8625v
PCH 1.0875v
Settings for 4.2Ghz
BCLK 200x21
Vcore 1.375v (huge increase for 200mhz so i left it at 4Ghz 24/7)
IMC 1.3125v
PLL 1.900v
PCH 1.15v
All other voltages set to minimum or auto. Both spread spectrums disabled. Obviously you will probably need to tweak these settings for your own setup.
I had to use a fixed voltage as i could never get the offset to work without the pc crashing. This also meant that all power saving options were disabled.