This message was posted on behalf of Intel Corporation
Thank you for contacting Intel® technical support. I will be more than happy to help you.
I understand that you are having issues with 720p and 1080p files in a 4K screen.
I do apologize for the inconvenience, but changing the screen resolution will not alter the quality of the files. For example, if a video was recorded in 720p, it will keep its quality even if you are playing it in a 4K screen. That would be the reason why you may see it smaller, or not as clear as you would expect.
About the video clarity, if you take a 4K screen and lower it resolution down to 1080p, it is usual that the entire screen clarity becomes affected since that is not the intended resolution for this particular screen.
In any case, if you want to change your screen resolution, you can just select an option in a range of supported resolutions. To do it, right-click the Desktop, click Graphics Properties > Display > General Settings; select the desired option under Resolution, and then click Apply on the bottom right.