Copy and paste the link of the YouTube video into the program and ensure that provided link has 4k quality in the YouTube player.
Check the option to download the full video file (audio plus video streams in one container).
Set the selector to merge audio and video streams into one physical file container. Merging of A/V streams will give you the best available quality of video stream. For some YouTube videos you will get up to 4k video resolution in the output (see 1).
Push download button.
Choose the output folder for you video.
Open the output folder with downloaded YouTube video (downloaded video file must be in best available quality, up to 4k quality)