Totally free, 5KPlayer from DearMob is a media-playing utility that is a lot more than a downloader—but it's got a heck of a downloader integrated. Promising no viruses, ads, or plug-in requirements is a good start. It is, sadly, one of the few I tested that asks for a registration of your name and email—you have to do that to get the full download function across 300+ sites. You may still use it if you don't register; I didn't and was able to download 4K vids from YouTube.
Multi-lingual 4K Video Downloader (4KVD) is frequently updated and features clear download links on the program's website; no ad traps here. The software does what it advertises in a simple interface: grabs videos up to 8K in quality and downloads to plenty of formats. Just copy a YouTube URL and click the Paste Link button to get started. 4KVD will even grab subtitles, entire playlists, and all the videos in a channel to which you subscribe. The sites supported are limited to the big names like YouTube, Vimeo, Facebook, and a few others, but covers most of what you need.