Your upsampled 4k looks better on my 4k monitors. There's more detail in your skin.
That being said, I'm not sure it's worth the extra rendering time and upload bandwidth for the marginal gain in quality. (did it take much longer to render/upload?)
I assume the problem in sony was smart resample? I hate that you can't set that to be disabled by default.
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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Edit: attached a 4x zoomed in photo, 2160 on left, 1080 on right, looks like more artefacts on the 1080 to me. In practice, I couldn't tell, and I had to try hard to find an example where I could identify a difference.
Thanks. Does seem to be better, but marginal at best.