1.6 V is far too high for that type of overclock, I'm assuming you've set the voltages on auto.
Try lowering to 1.5 V or so (absolute recommended
maximum), but I'm not sure if you can reach 4.0 GHz that easily with an H50 on a C2 955.
This guide explains everything well, and all the terms etc.http://www.overclockers.com/forums/showthread.php?t=596023
If you're serious about a 4.0 GHz overclock you will need to learn how to do it properly, otherwise you could damage your CPU.