There are many online video downloader right now and some of them can download 4k video from Youtube or Vimeo.
Here's a easy use one and its tutorial for download 4k videos with highest quality.
Just 3 steps to do it.
Step 1. Download and open it.
Step 2. To copy and paste the video URL into the "Analyse" box. Choose suitable video size and resolution.
Step 3. Click "Download" to start!