Those claiming more detail and better colour have to be seeing something artificial. The original footage is not 4K. If there are highlights being blown and detail vanishing it has to be in the encoding process.
When I watch upscaled DVD using the upscaling system on my HD TV it looks better than the upscaling system on my cheap DVD player, but the PC upscaling looks even better. The picture is sharpened and various other filtering applied, which obviously isn't there on the DVD. I suspect the same is happening here when the source footage is being upscaled to 4K. That may also be why some people are seeing better results than others - the differences may be in the upscaling or downscaling being performed in software or hardware when viewing, rather than the video itself. Remember that You Tube uses hardware acceleration when available, and the quality of graphics cards and drivers can vary considerably.
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