I would like to experience what 4K or 8K videos looks like, where to download?
Sample videos are enough, no need for full-length videos.
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First off, realize that 4k video is generally 4096 x 2304, which most monitors will not support it directly - even high end consumer 30" displays today only display 2560x1600, and the 15" Apple Retina display is still only 2880x1800. The video will generally be down-sampled to display properly, similar to the way if you tried to play HD video on a non-HD display.
There's a sample 4k video on Wikipedia (make sure to click Full Resolution" to get the 4k file)