You quite possibly could have maybe fried one of your cores, i did that with my last CPU.
I would say maybe downloading some software of some sort to test your CPU, and maybe try reinstalling it, and make sure there are no issues with the contacts, like dust or dirt. and make sure the CPU is properly placed in it's socket.
I would recommend running the Intel(R) Processor Identification Utility to determine if the processor is being properly detected by the system.
You can download the Intel(R) Processor Identification Utility at:
If this tool reports only 3 cores on the processor, then I would recommend installing the latest BIOS version for your motherboard and if possible testing the processor in a known working and compatible motherboard to see if it causes the same behavior.