Uhm... pardon me, what what is your monitor setup / resolution?
All that "4GB" means is that it has more video RAM. Unless you've got a resolution higher than 1440p, you really don't need a 4GB card, and are just wasting money.
And I know that you don't want to hear it, but I'm saying it anyways: if you buy a 680, you will be wasting your money. A 680 will only be 5% faster than a 670, and that number lessens after overclocking. Meanwhile, it's 25-30% more expensive. How does that make sense?
You'd be paying $100 (or more, for the 4GB version) extra to DO NOTHING BETTER. There's not a game that the 680 can max with playable framerates that the 670 can't.