4.0Ghz OC can probably be done on stock voltages. Keep in mind, the 2500 overclocks itself underload to 3.8.
Im not positive on your particular Motherbaord, but there are plenty here very familiar with them. Just hang on tight and Im sure someone will be along shortly to help.
But, essentially all you need to is find you CPU multiplier setting, and change it to manual. Then change it to x40. And thats it.
Depending on your particular chip, you may also need to tweak the voltages a bit, then again, you may not.
And as always, you will still need to test it for stability