You need more than a 2GB frame buffer for 4K gaming (SLI doesn't double the memory capacity, so you are still limited to 2GB). AMD wasn't joking when they said you need 4GB for 4K gaming.
I recently dropped my SLI numbers from the charts to make them smaller (they are too big still) and the GTX760 and GTX770 got killed at 5760x1080 in Far Cry 3. Since there are more pixels being rendered at 4K, I can say with certainty that the cards would again fall on their face. Here is the last Far Cry 3 chart with SLI included and you can see that SLI does squat for 5760x1080.