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Where are you seeing the CPU core information? What operating system do you have installed?
Check in the bios and see if all 4 cores are enabled and update the BIOS to the latest version.
You also need to realize that having a dual or quad core CPU does not automatic mean all core will be used.
If the operating system is really seeing one core I suggest you to reinstall the operating system or if your system was running fine before you can try a system restore before reinstalling the operating system.
I hope this can help.