Does YouTube allow downloading 4k videos to a desktop computer, phone, or tablet?
Unfortunately, YouTube does not currently allow the downloading of 4k videos at this time (at least, legally through the YouTube app or website).
The current choices for video resolution include Ask each time, Full HD (1080p), High (720p), Standard (480p), or Low (144p).
That means Quad HD (1440p), 4k UHD (2160p), or 8k (4320p) video resolutions are not currently available to download, although there are videos with those resolutions available for viewing on YouTube.
You can set your YouTube video resolution preference under the Settings > gear icon. The gear icon can be found in the upper corner of your Profile screen on apps, or in the Profile settings in the upper right corner of Chrome or other browser.
YouTube Video Quality Preferences
Also Read: What is 4k Ultra HD TV resolution?