Things are becoming interesting here. Not everyone has a 4K-ready PC, but if your TV is 4K, you want to get the best possible image. To do that, you want your PC to always run a 4K resolution, after which we can use a few tricks to scale up your games from a lower resolution – similar to what the Xbox One X and PS4 Pro do. You get a better overall picture than just running your PC at 1080p without having to upgrade your graphics card.
First right-click on the Windows desktop and select Display Settings. Scroll down to Screen Resolution and set it to 3,840 by 2,160 (it says "Recommended" in parentheses beside it). This ensures that your PC outputs a 4K signal.
Then start a game and enter the video or display settings. Ideally, it has a setting called Resolution Scaling (sometimes called Render Scale or something similar). This setting is usually a percentage value and ensures a lower resolution of the game, while other UI elements are kept at a super sharp 4K.
For example, you set the resolution of your game to 3,840 by 2,1
Some games may have even more options to bridge this gap, such as Temporal Filtering from Watch Dogs 2 or the adaptive resolution from Titanfall 2 – although these are now slightly less and farther.
Look around the settings and see what you can find – make sure that both Windows and your game are set to 3,840 by 2,160 before you adjust these settings for the best results.