You don't "need" anything, you can just keep what you have. Monitors are a personal choice and what you should get depends on what you are trying to achieve. Do you want a nicer picture, faster framerates, bigger fonts, less input lag, what? Without knowing what it is you are wanting from a new monitor no one can make a serious suggestion.
For me personally my main goal has always been to play any game I want at maxed out or close settings without having to buy a top of the line graphics card so I have stuck with 1080P until quite recently and now have gone for an ultrawide 2560 x 1080 for the same reason. Competitive E-sports gamers favour very high frame rates and low input lag over resolution and colour accuracy, I favour IPS 1080P screens for the wide viewing angle and nicer picture and am quite happy gaming at 60 fps, it's a personal thing.