No single GPU exists that can run most games at 4k with ultra settings and never drop below 60 FPS. 2560 x 1440 is the sweet spot right now. If you get one with a refresh rate above 60Hz, even better.

The downside to using a tv as a gaming monitor is they don't have the tech to make gameplay as smooth as it could be (see vsync, freesync, etc.).

4k is definitely super clean looking (depending on how good your eyesight is) but 1440p is certainly good enough.