Open your task manager and go to the Performance tab. At the top under cpu usage history, you should have a balck box with a green grid for each core you have. If it only shows 3 and you have a quad, your CPU may be defective.
There is also a setting in BIOS to turn on and off individual cores. One of your cores may be turned off.
My brothers AMD 6 core had one turned off in the BIOS when it was first put together. Found the setting and enabled it.
Have no idea why 1 core was disabled on a new motherboard but it was.
Computer type PC/Desktop System Manufacturer/Model Number Home Built Desktop By DataTech OS Windows 7 Ultimate X64 SP1 CPU Intel i5-2550K, Differing ~4.4-4.8GHz No built in GPU Motherboard ASUS P8Z68-V PRO/GEN3 Memory 16GB G.Skill Sniper 2133MHz 2x8GB Graphics Card ASUS GTX650TIB-DC2OC-2GD5, (650TI Boost) Sound Card Onboard Realtek 5-1 Monitor(s) Displays Samsung P2570HD Screen Resolution 1920x1080
Keyboard E-Z Eyes, bright yellow keys with large characters Mouse steelseries SENSEI Laser Pro Gaming PSU Corsair HX650W Case Inwin Dragon Rider Cooling Hyper 212 EVO w/two Noctua fans, push-pull, @1300 RPM Hard Drives Smasung 840 Pro 256GB for OS, 500GB Seagate Constellation (Enterprise drive) for Data Internet Speed 48-51Mbs Mbs down, 11 Mbs up Xfinity Cable Antivirus Norton Internet Security 2013 Browser IE 10, Opera when needed Other Info 4 case fans, LG BluRay-RE, ASUS DVD-RW, Mr. Fusion power supply, 1.21 gigawatts.