Regardless if you have the Phenom II x4 965 you linked or the Phenom II x2 550 CPU-Z is telling us you have, it is not running at 4Ghz.
To date, the fastest AMD CPU runs at 3.4Ghz stock, your x2 550, 3.1Ghz.
The 4Ghz number refers to the Hyper Transport speed.
Your current speeds are lower than their rated because the CPU is in a low power state.
Once you start using some CPU power, the multiplier and voltage will go back up and your speed will return to the standard 3.1Ghz.
Anything that loads the system will snap it out of the low power state.
IDK, try loading a bunch of apps at the same time or something.
Be sure to have CPU-Z open when you try and you should see the Multiplier+Clock Speed jump when you do.
After everything is done using the CPU, it should drop back down into the low power mode again.