The 4GB vram is only useful if you'll be gaming in SLI at triple screen resolutions. If you won't be doing that, just stick to the 2GB card.
You couldn't use 4GB of VRAM. And toms did an article recently about a 6GB 7970 and it had no performance advantage. So its not worth it. Although for only $27 more its not that bad, but before 4gb is needed we will be a generation or 2 more advanced in GPU's and the architecture at that time will destroy this 4gb card. Stick with the 2GB.