If you wanna render at 4K + ENB just for screenshots, I believe Shift + F12 will toggle it on and off. Normally on my 570 I can get completely playable framerates rendering at 4K with the effects turned off.
Is this your own custom ENB? Looks really great.
Yeah, I have to disable it if I'm playing anything higher than native (but that's just for screenshots mostly). I can play at my native res with ENB just fine. Seems to be around 30-40 FPS. But it's a complete slog at 4K and above. These screenshots are captured at 5120x2160 or higher (the rat one was 6400x2700).
On the other hand, at 5120x2160 WITHOUT ENB, it's almost a solid 60 FPS. Though that's partially because ENB requires you to disable in-game SSAO, DoF, and AA. So the difference when enabling ENB is even more staggering.
Anywho, this is Midhras' ENB that he sent me. I tried the most popular one on the Nexus (
Wickfut's) but it's extremely dark. Otherwise looks good. I just didn't have the patience to tweak it.