First you might want to list all of your system specs it might help or it might not.
After a bit of research I have found that a lot of other people have had this problem, so I will list some of the things that were recommended to them or if they solved it.
First check the device manager and see if it says there are 4 cores, if it just says 1 core then it could be something in the bios but if it says 4 then windows must be disabling the others.
One of the recommendations was to go into msconfig then go into the boot tab then go into advanced options then click number of processors and put 4 in if you can.
one person recommended looking up your CPUs stock voltage and manually setting it to that in the bios.
A lot of people usually recommend that you keep looking in your bios and even check the motherboards manual to see if there is anything you need to enable or disable.
One person had started taking there computer apart then needed to put it back together and it started showing all cores so i would guess that they might have done something like reset the heat-sink and it may have changed how the CPU was sitting in the slot but you may want to save this as one of your last choices.
Maybe you could take your windows disk and run the repair option or what ever it is and see if its just something missing from windows.
The last thing is that someone recommended reinstalling windows if all else failed so this may be your very last resort though I would keep looking before doing so.
I will keep looking to see if there is some perfect solution.
