MSI added a 4GB SKU of their GeForce GTX 680 Twin Frozr III which can be found as model: N680GTX Twin Frozr 4GD5.
The card has standard 1006/1058 MHz core/GPU Boost and a seprate SKu for the OC model at 1058/1124 MHz core/GPU Boost. The 4 GB of memory comes clocked at a reference 6008 MHz (GDDR5-effective). The new 4 GB models are likely have a 10 to 20% price premium.