very few Q6600's ever reach 4Ghz stable. i've owned three, my brother had one, and both my current housemates have one each. NONE of them can go past 3.6Ghz stable on air. That took upto 1.45v to achieve on two of them, two others took 1.5v+ (and 85C load temps on a TRUE120)
All those reports of 4GHz Q6600's were from people who did nothing but post screenshots proving they'd hit windows - very, VERY few of those people could prove their systems were stable at those clocks.