To quote LightORamaJohn in a post he made on the forums.

The board you are talking about is a g3 controller. These CTB16PC boards are marked “CTB16PCG2 – V4” which refers to the circuit board rev level. The main differences from the previous generation are faster processors, higher current accessory power supplies, ghost loads and much better zero cross detection — obviously we also moved to surface mount technology to keep the price stable. Additionally, the firmware has been upgraded to support LED curves. We decided to call these g3 since there have been a few generations of the original PC controllers. We should have changed the silk-screening to avoid confusion. Sorry about that. The picture attached is a g3 CTB16PC.