How do i unlock 4th core on hp pavillion m9250f?

If you have a quad and only 3 cores are working, then you may have a defective CPU. Is the CPU a Q6700? Have you checked if it's possible to disable cores in the BIOS?

cpu is a 6700 . unable to disable cores in bios .

According to Intel's website the Q6700 is a quad core. If it truly is not showing all four cores you might have gotten a screwed up CPU. When you go into task manager under performance how many core graphs does it show? Three or four?

it only shows 3 graphs . if my cpu is screwed how much will a new one cost ?

Well it's a 775 socket and they're old and I'm guessing this computer isn't still under warranty? If it is by some chance still under warranty HP should replace it for free either with another Q6700 or something else. If it is not still under warranty then you'll have to buy a new one yourself. Problem is it might be hard to get a replacement since they are out of stock and de-activated at New Egg and other computer stores. Check Hp's website and find your model computer and check the CPU support list. Maybe it will accept a newer version of Intel CPU.

You can also try reinstalling the operating system. It is possible that something in the OS became corrupted and is preventing the 4th core from being recognized.

This is a long shot, but it is worth a try since the cost is $0. Just make sure you backup all important files on your C: drive before doing a fresh reinstall.

i think i will up grade my processor , one that will be accepted by the motherboard , a Intel Core 2 Quad (Y) Q9xxxx. think that will solve the problem ?

If the motherboard is compatible with the processor then yes. But again it's best to check your model's CPU compatiblity list to be sure it will work. It has to be the right company (Intel in this case), right socket and right TDP or it won't work. You'll spend a 100+ dollars on a CPU paper weight if it's not compatible with the motherboard.

i checked with hp website and the cpu is listed as an upgrade . when i find one and install it , will let you know how it goes .

you can also just keep the current cpu as performance wise if it works on 3 cores you'd actually not see much difference spending money on a new cpu.

in the meantime, you might benefit from removing the cpu and heatsink, clean and inspect, use high quality heatsink paste, reassemble.