After setting the frequency desired in the BIOS, its worth testing the stability of the overclock with a program such as Prime95 (run for hours) or intel burntest (20+ runs) to see if there is any errors reported.
To increase the stability a tiny bit more Vcore can be added in the voltage section of the BIOS.
At 4Ghz I would imagine nothing needs adjusting, but at 4.5GHz more will be required, the other thing about the motherboard is that it may auto adjust this value itself to suit (called serialVID)
For example if you go into the BIOS and make a note of the vcore reported, then set the multiplier to a higher number (40+) then save and exit the BIOS, if you go back in you should see the Vcore has increased all on is own.