Once you have changed your appropriate settings simply load up your favourite game and select 3840x2160@240hz which should now be an option for you alongside 1920x1080@240hz. You will be rendering the image at 4k and squeezing it on a 1:1 pixel ratio (x4 sampled) and thus will have the exact same fps as any other 4k setup. Nvidia use a high quality filter so remember to disable all forms of antialiasing and post process effects which hide the sharpness of 4k.
You may only run into one issue any game not showing the option for 3840x2160 simply requires you to go into the Nvidia control panel to resolutions tab and scroll up and look for the two 3840x2160 entries. Only one will work it will be the shortest of the two saying only 3840x2160 this brute forces the desktop to 4k and thus forcing any game to render native 4k.
My last fix here is sometimes the mouse can become bugged and trapped in a quarter of the screensize. This will lock your cursor leaving you unable to select quit in some games so to prevent this right click your exe for the games and in the compatibility tab tick the box saying disable high dpi scaling. I have yet to find a game that does not work but many have poor UI scaling making it not feasable in every case. But it just works!
