The most obvious difference between AMD's Phenom II x3 and x4 processors is the number of cores: the x3 has three and the x4 has, unsurprisingly, four. However the chips themselves are actually the same hardware, with AMD locking out one of the cores in the x3 variants. That presents an almost irresistible target for modders, of course, who have shown that with the right motherboard you can turn your Phenom II x3 into an x4.
The hack is reliant on the "Advanced Clock Calibration" option found in some motherboard BIOS settings. Switch that to Auto and hey presto the fourth core of your Phenom II x3 is spotted. Compatible motherboards include certain Biostar models, though as ever you could seriously ruin your CPU and/or system if things don't go according to plan.
AMD announced the Phenom II x3 range earlier this month, in the shape of the Phenom II x3 710 (2.6 GHz, 1.5MB L2 cache, 6MB L3 cache) and Phenom II x3 720 BE (2.8 GHz, 1.5MB L2 cache, 6MB L3 cache). They use the new AM3 socket, as well as being backward compatible with the AM2+ socket.