When streaming to twitch (i.e. usually when I play online), I play at a lousy 1920×1080 (downscaling by a factor of 2.25, so 852×480, for the stream). My processor isn’t that strong and the streaming uses a software codec so I need keep my resolution down as much as possible. It looks bad but well… Shadowplay isn’t an option for live streaming (terrible quality at recommended twitch bitrates), and also using a separate encoder allows me to use Shadowplay for high quality recording simultaneously whilst streaming. In any case, it would be nice if NVidia would spend some time at improving their encoder for low bit rates.
For normal gameplay I can play the game quite comfortably at 2560×1440, 60fps, with all the settings maxed out (except for “Advanced Graphics”), and anti-aliasing disabled. It looks very nice, and IMO better than 1920×1080 with AA.
At 3840×2160, it still works at about 30fps provided that grass is set to normal (and AA disabled of course – AA makes absolutely no sense anymore at 4K). But GTA looks really gorgeous in 4K, especially all the little details in the distance and in close-ups. When taking screenshots from the Rockstar Editor, I usually set it to 3840×2160 with all the settings up to their maximum level (including “Advanced Graphics”), keeping anti-aliasing disabled. This only exceeds the memory limit on my card by a very small fraction. But the game plays at less than one frame per second, so it’s really only good for taking high quality screenshots.
Other less demanding games handle 4K with max settings without issues (like Saints Row).