I know there are hundreds and thousands of YouTube online videos downloaders, but is there any program that supports downloading 4K Ultra HD YouTube videos and saving them as .mp3 music files?
As the 4K technology is advancing day after day, a large range of YouTube video downloaders support downloading 2160p UHD online videos from YouTube, such as OpenMedia's 4K Video Downloader, Digiarty's WinX YouTube Downloader (which is a 4K video downloader) and Open Media LLC's 4KDownload.
As for converting 4K YouTube to MP3, AAC, AC3, WMA, AIFF, CAF, DTS, OGG, AU, RA, WAV or FLAC, ALAC audio, you can try WinX HD Video Converter Deluxe. It will download online 4K, 8K and 3D videos from YouTube and in the meantime, convert MP4 to MP3 music, extracting the audio part only. The audio file can be played on your iPad, iPhone or Android. It can also convert 8K to 4K, 4K to 1080p, 4K to 720p, 4K to 480p, etc. at fastest speed thanks to its multi-core CPU and most advanced hardware acceleration tech (incl. Intel QSV, Nvidia CUDA/NVENC, etc.). No toolbars, no adware, no malware.
Step 1. Download the 4K YouTube download converter to your computer, and execute the installation.
Step 2. Click "YouTube URL" button to add the original 4K YouTube video to this tool.
Step 3. "Paste and analyze" the 4K YouTube video URL and tick a resolution option listed, check "Auto Convert" and click "OK".
Step 4. Under Output Profile Category, click "to Music", select "MP3" as your output format and OK.
WinX HD Video Converter Deluxe, with 410+ ready-made profiles, can also convert 4K YouTube videos to AVI, WMV, MPEG, MKV, M2TS, iPhone (iPhone 6 Plus included), iPad (iPad Air 2 and Mini 3 included), Android, PSP/PS3, Apple TV, Chromecast, WP8 and so on.
Step 5. Make any other advanced settings if necessary, and hit RUN button.