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I downloaded it with youtube-dl because I cannot stand the excessive adverts on Youtube these days. During the download it grabbed this video file:

  Input #0, matroska,webm, from 'New - Mars In 4K-ZEyAs3NWH4A.f313.webm.part':
  Metadata:
    encoder         : google/video-file
  Duration: 00:10:08.41, start: 0.000000, bitrate: 4436 kb/s
    Stream #0:0(eng): Video: vp9 (Profile 0), yuv420p(tv, bt709), 3840x2160, SAR 1:1 DAR 16:9, 29.97 fps, 29.97 tbr, 1k tbn, 1k tbc (default)
So yes it's 4K.
> I downloaded it with youtube-dl because I cannot stand the excessive adverts on Youtube these days.

Can't you use an ad-blocker?

Or, you know, pay for the premium version
You're not really paying for the ads, you're really paying for an incredibly crappy music streaming service (Youtube Music).
The music service is different from ad-free youtube. As for YT music it's the same as all the other music service, they all have the same catalog.
Mine goes up to "2160p" but since the source photos are stitched together, I'm not sure what their total resolution would be (probably varies based on what/how many were stitched together).

It's basically some pans across high resolution composite images. Quite cool but not captured video. As they mention, there's not much moving there anyway but it will be cool when someday we can look at high def (or even stereoscopic) video from these remote locations.

You can get stereoscopic images from the Curiosity navigation cameras. You could do a much better job of correcting the lens distortion, but I made an example in 2012 (I decided to make the sky blue as a stylistic choice at the time, for which I was rightly criticized as Mars does not have a blue sky and people might think the images were from Earth): https://blog.mikebourgeous.com/2012/08/06/curiositys-view-of...
I was experiencing that on Safari, then switched to Chrome and it now shows 4K.
Confirmed. Once I switched to Chrome, I could see the 4K quality option.
2160p also shows up for me on Firefox & Brave.

Golden rule: don't use Safari for Youtube.

Is this about VP9 support?
From Wikipedia regarding VP9[1]: "Parts of the format are covered by patents held by Google. The company grants free usage of its own related patents based on reciprocity, i.e. as long as the user does not engage in patent litigations."

Of course Apple refuses to add VP9; Google demands that Apple would never sue them over any patent. Any. It looks like Google/YouTube caved in, and has/will start encoding (the most recently uploaded) videos in AV1 codec, which will work on tvOS. Hopefully that will translate to Safari on macOS and iOS for those who use it.

[1] https://en.wikipedia.org/wiki/VP9

Yes.
In 9/10 cases you don't see the difference anyway unless you are actually on a very large screen and you don't mind wasting the extra computing power.
If you're talking about a beautifully mastered Blu-Ray, then that might be true.

But given how aggressively YouTube compresses videos, watching in 4K can provide a huge improvement over 1080p even if your screen only has, say, 1,600 lines like my MBP does, instead of the 2,160 lines that 4K actually produces. It's really about the clarity of textures and how lifelike they are -- it's very noticeable. Of course, this also assumes you have good vision.

I also don't think anyone's too worried about "wasting computing power" here for a short video.

That's curious. My observation has been that most of the streaming services have really high compression ratios with 4k streams. With videos like this one, which are built from still images, I imagine you still get significant improvements in quality, but for content that isn't so easily compressible (e.g. action movies), I've seen significant effects from compression in 4k streams as compared to "regular" 4k source material.

As far as "wasting computing power"... I hear some people play video games while watching movies... on the same machine. So that's probably what that is about.

That's exactly what I'm saying, though.

Because YouTube (like other streaming services) aggressively compresses both 1080p and 4K, but the 4K version still has a much higher bitrate, watching the 4K stream on a 1080p monitor will be significantly better quality than watching a 1080p stream on a 1080p monitor.

I'm not saying you won't see any compression artifacts at all, of course if it's an scene you will. But the 4K version is still noticeably way better than the 1080p one, even in an action scene. It's a higher bitrate so there's more detail no matter what.

Yeah, what I'm saying is that because the 4k streams are trying to convey more entropy than the 1080p streams, I've observed that even though their bitrate is higher, the resultant compression effects are far more severe, to the point where the overall experience is better with 1080p, sometimes even if you have access to the full 4k resolution. I've seen similar effects with audio content (after a certain point, increasing sampling rate significantly more than bitrate has a deleterious effect).

From an information theory perspective, you are obviously correct that there is more information in the 4k streams and therefore you will get more detail from them. My observation however has often been the opposite.

My attempt to rationalize leans towards notions that the lossy compression techniques we use with video codecs still have significant impedance with the "compression" in our neuro-opthalmologic system.

Oh now I understand what you're saying -- that makes sense. It's not what I've observed, but I definitely can imagine how settings for the compression, or what the algorithm concentrates on, could make that happen. I'll have to pay close attention now! And your rationalization probably makes sense -- I read the h.266 codec is supposedly focusing specifically on perception improvements in 4K/8K, so it may be addressing what your'e perceiving specifically.

Funnily enough, this reminds me of what I did with JPEGs on websites when Retina first became a thing. Instead of serving up different assets depending on resolution, I discovered that serving up a 2x resolution JPEG to everyone, with really crappy quality, was superior to serving up 1x with high quality. For 1x screens, blockiness was inherently shrunken because of JPEG's fixed block sizes, and for 2x screens the low quality was harder to perceive because it was at a small scale anyways. How compression interacts with scale/resolution is not a necessarily intuitive thing.

That's a brilliant conclusion on the JPEG hack, and yes, that's precisely what I've observed. You really notice it if you compare to even 1080i with high bitrates (OTA HD TV for example). It's weird.
I believe it's worth watching 4k even on a 1k monitor, because due to chroma sub-sampling there is actually information visible even at the lower resolution.
Do the video decoders take advantage of this? I was under the impression they did not.
For me, it lets me choose both 2160p(4K), 1440p(HD), and 1080p(HD).
I have a 4K option in the youtube menu.

At one point in the video, there is a 1.8 million pixel image where they discuss and show zooming into the image while preserving detail.

If you are trying to watch this on a phone then the YouTube app is limiting the maximum resolution to the resolution of your phone.
For me, YouTube is doing 2160p.
Goes up to 4K for me, are you on mobile?
MacBookPro with Safari.
Safari hasn't supported >1080p in years (because it blocks VP9 and Youtube has stopped offering higher resolutions in H264 that Safari supports)
Blocks VP9? Maybe it doesn't support it but blocking would imply it is built-in but forcibly disabled.

It does look like they're testing it since it's in tech preview [1] so once it's relatively-bug free they'll probably release it.

[1] https://developer.apple.com/safari/technology-preview/releas...

Yup. Golden rule: don't use Safari for YouTube (or really for video content in general).