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 4x4GB Graphics Card ASUS ENGTX460 DirectCU/2DI/1GD5 GeForce GTX 460 Sound Card Onboard Realtek 5-1 Monitor(s) Displays Samsung P2570HD Screen Resolution 1920x1080
Keyboard Old, beat-up Dell USB From 10 yrs Ago Mouse Gigabyte m6900 wired PSU Corsair HX650W Case Inwin Dragon Rider Cooling Hyper 212 EVO w/two Noctua fans, push-pull, @1300 RPM Hard Drives Crucial M4 128GB for OS, 750GB Seagate MomentusXT for data, 500GB Seagate Constellation for storage Internet Speed 8-19 Mbs down, 3-4 Mbs up Comcast Cable Antivirus Norton Internet Security Browser IE 9, Opera when needed Other Info 4 case fans, LG BluRay-RE, ASUS DVD-RW, Mr. Fusion power generator with flux capacitor, 1.21 gigawatts.