Overclocking video cards doesn't do
that much. It's still just the same chip. Overclock the balls out of a 480, and it still won't keep up with an underclocked 680.
Here's a benchmark showing the EVGA 980 Classified, perhaps the most aggressive version of that card, and it is aggressively overclocked:
http://www.hardwareluxx.com/index.php/reviews/hardware/vgacards/33340-test-evga-geforce-gtx-980-classified.html?start=11
I linked the article rather than the graphs because they are interactive and can't be linked here, which means you'll actually need to click the link and review it in order to see what I'm referencing.
When you click the link, you'll see that an aggressively overclocked 980 Classified in Battlefied 4 on Ultra at 4K gets only 47.7 fps, and it drops frames down to 41, with AA down to its lowest setting. Applying more drops you down into the 30s on average, going into the low 20s.
Given that an overclocked 980 Classified is as fast as the 980s get, and that the 980 is currently the fastest card, it's pretty clear that no single GPU card in existence right now can deliver smooth gameplay in games like Battlefield 4 at 4K on Ultra settings.