I am able to get the 4th core unlocked with just a simple ACC and HT Link Speed change using my Biostar TA790GX A3+ motherboard, but it is extremely unstable. The computer just starts acting weird and crashing. Obviously there is a stability problem with the 4th core..but I'm not yet convinced that it is just not going to work.
Here are my motherboard options (thanks to PcCI2iminal for the image)
CPU Vcore: Auto (Tried a slight bump)
CPU-NB Voltage: Auto
Core FID: x13 2600 MHz
Core VID 1.3000 V
Core DID: Divided by 1
NB FID: 2000 MHz
NB VID: 1.1000 V
SVID Override: Disabled
HT Link Speed: 1.6 GHz
HT Link Width: Auto
Advanced Clock Calibration: All Cores
Value (All Cores): -2%
Core Leveling: Auto
CPU Frequency Control: 205MHz (will only boot into Windows 205 or less).
What can I try in order to make this 4th core run more stable?
Go back to stock speeds to ensure everything is working fine.
Remember, increasing the FSB also increases the speed of your memory as well as the PCIe slots. What is your memory type? Is it made for overclocking? Have you also overclocked your GPU as well? I would set everything back to stock speeds and ensure that works. Then, try increasing the FSB again in small increments. You may have to loosen the timings on your memory when you do increase the FSB, but it also depends on what brand of memory you have.
When you say you know the first thing.. what would that be? How to change FSB or something? I agree with isamuelson and suggest setting everything back to stock speeds to make sure the chip is stable again. Run Prime95 for a good hour or two to check temps and stability at stock. If they are, so as isamuelson has said and increase your FSB a little at a time, each time checking stability.