CMOS Setup Utility Main Menu --> MB Intelligent Tweaker(M.I.T.) --> Advanced Frequency Settings --> Advanced CPU Core Features --> CPU Clock Ratio [33X]
Change the CPU Clock Ratio from its default of [33X] for the Intel Core i5-2500K to [40X] to get to 4.0 GHz.
CMOS Setup Utility Main Menu --> MB Intelligent Tweaker(M.I.T.) --> Advanced Voltage Settings --> CPU Vcore [Auto]
You don't necessarily have to change it from its default of [Auto]. The voltage will vary with the CPU speed which varies with the CPU load. If you change this setting to [Normal] you can use the Dynamic Vcore(DVID) setting to make changes to the voltage by using an offset voltage which is the preferred voltage adjustment method.
You can download and use the freeware utility CPU-Z to monitor the CPU's Core Voltage and speed.
You can download and use the freeware utility IntelBurnTest v2.52 to test for CPU stability. I prefer this tool over Prime95.
Use a CPU temperature monitoring utility to keep an eye on the core temperatures.
You should also monitor the CPU's core temperatures to ensure that it doesn't get hot enough to trigger the CPU's thermal protection. Try to keep the temps under 75°C.
If you ever plan on using more than one graphics card in the future then the GIGABYTE GA-Z68MX-UD2H-B3's micro ATX form factor is awful.
The secondary graphics card would interfere with the many I/O headers (i.e. F_AUDIO, SPDIF_O, F_USB5, F_USB4, F_USB3, F_USB2, F_USB1, F_USB30, F_PANEL, CLR_CMOS) along the edge of the motherboard.
If you never intend to add a second HD 6950 in CrossFireX mode then you shouldn't encounter this problem. You can also use single slot width graphics cards as a solution.
If you intend on using this motherboard for video transcoding, using the iGPU on the Sandy Bridge CPU, its 6+1 phase power VRM design sucks. The +1 phase is used to power the iGPU and is considered low end and unstable.
If you're intent on overclocking your Sandy Bridge CPU this motherboard is not that great at doing that either.