The technology is refered to as SpeedStep. It drops your clock rate down to 1.6 when the cpu is being lightly used to save power, then ramps it up to your designated clock rating when the cpu begins to work harder.
So yes, when your playing a game the cpu should speedstep up to 4ghz. You can disable this in the BIOS. Go to the advanced BIOS, then the Advanced tab. Under the Advanced Tab there should be CPU configuration, then under the new screen you get to from choosing that option, scroll down to CPU Power Management, the speedstep enable/disable will be in the new window onces CPU PM is selected: http://www.legitreviews.com/images/reviews/1767/asus-p9x79-bios-19.jpg