With the popularization of 4k televisions and displays, came a version of the BluRay disc format supporting 4k resolution video, as well as HDR pixel format. This to complement the already-existing "full-HD" (ie. 1080p) BluRay discs.
In theory this is great. If you have a 4k display and you play eg. a 4k demonstration video on YouTube (at a very high bitrate), the resulting picture quality can be amazing: Extremely crisp and sharp images full of tiny details, most of which is less visible in a "mere" 1080p version of the video. Many of these "4k demo" videos are optimized to showcase the advantages of the extra resolution.
Thus, "4k HDR" BluRay discs have become more and more popular and available for purchase, alongside the older regular 1080p BluRays. At a premium, of course.
The thing is... in the vast, vast majority of cases you aren't getting what you are paying for. In a few cases you are, ironically, getting a worse version of the movie, in terms of image quality, compared to the normal 1080p BluRay version. On top of that you might also have to struggle with technical issued when dealing with "4k HDR" BluRay discs.
One would think that a 4k resolution release of a movie would use extremely high-quality high-resolution scans of the original movie material. This can be especially enticing with older movies, especially the cult classics: Finally you will be able to see them in their full original movie-theater-quality glory, rather than in blurry VHS quality, or even blurry DVD quality that's full of visible compression artifacts.
With new movies released in the last half a decade you might, might, get a slightly better-quality picture. However, even in these cases the difference compared to the 1080p version can be minimal or even pretty much non-existent. The problem is that the source material itself is usually not of much higher quality than 1080p. Maybe a bit, but not a lot. And a significant portion of the original quality may be lost in the conversion to BluRay and its video compression format. (A recent movie that has been shot digitally might have a higher chance of giving you extra detail in its 4k release, but even then the difference tends to be very minimal, almost imperceptible.)
With older movies, like the all-time classics and most popular movies, you are usually out of luck, and in these cases the "4k HDR" release can be for all intents and purposes a scam: You are not getting any better image quality than the normal 1080p BluRay. They are selling you a brand, not actual higher-quality content. It's like when they sell you tap water in a plastic bottle with the picture of snowy mountains and blue lakes: It makes it feel like you are getting better water, when in fact it's the exact same thing you get from the closest water faucet.
Indeed, it's extremely rare for them to go back to the original movie reels and re-scan them in higher quality and higher resolution for a "4k" BluRay release. What they usually do is just take the same material they already have (which they might have previously published on BluRay, or even just DVD) and upscale it with perhaps some minor sharpening applied. At points the post-processing sharpening might give the illusion of slightly better image quality, but it actually isn't: You are getting something that wasn't in the original movie, but something that was manufactured later.
And the "HDR" part of it? Most often than not the only thing they do is to brighten the image, and that's it. As mentioned, this isn't a re-scan of the original movie reels, just taking the old existing BluRay/DVD video and upscaling it and changing its brightness. You are not getting any new and better material. You are just getting the same old material with a little post-processing applied to it. (Post-processing which, by the way, you could apply yourself in real-time with many video players, if you really wanted.)
However, it can be much more problematic in some cases than just slightly artificially enhanced sharpening and brightening.
You see, a few of these "4k HDR" releases go beyond just applying a light post-processing filter to the image. Some of them apply a sharpening AI filter which uses AI technology to add detail to the picture that wasn't there already. A bit like Photoshop's Generative AI.
This has two major problems: Firstly, these added details were not in the original movie material. They were created afterwards, for the "4k BluRay" release. In other words, you are NOT getting the original movie in higher quality. You are getting a MODIFIED version of the movie, where the "higher quality" has been generated, not scanned from the original reels.
Secondly, and most importantly: The result of this generative AI can be absolutely horrendous. If you google for it, you'll find plenty of examples. Faces and hands that were small and blurred in the original movie may become horribly distorted in the AI-enhanced version, to the point of being absolutely hideous. Faces and other details may become very artificial-looking, and nothing like in the original. The generative AI may add strange details around shapes, making them look like they have been filmed against a greenscreen (even when they were originally filmed completely live, without any greenscreens).
In other words, when you buy an older movie in "4k HDR" BluRay version, you may be literally getting a much worse version of the movie compared to the regular "full-HD" 1080p BluRay. The changes to the picture may be so drastic as to outright destroy its quality.
Then, on top of that, and as mentioned at the beginning, you might also run into technical problems with the "HDR" part of the disc (something you'll likely not encounter with the normal 1080p BluRay version).
For example, I have a PS5, which supports "4k HDR" BluRay discs. However, I have many technical issues viewing the few such discs I have.
It may be a problem with the PS5 (and might not be such a problem with dedicated 4k BluRay players), but if the disc has the video in "HDR" format (which all of the ones I have do), it just demands the TV to support HDR as well. Or, to be more precise, it won't refuse to play the video, but it won't do any sort of conversion to "non-HDR" displays, in case your TV or display doesn't support HDR.
The TV I have the PS5 connected to supports HDR... technically. It's one of those cheap-ass 4k TVs that technically support it, but not really. The difference between normal video and HDR video is very small, even non-existent.
The problem with that is that if I leave HDR enabled in the PS5, games tend to not look so good, because the HDR support in the TV is so crappy. Thus, I have it disabled by default.
However, for these "4k HDR" BluRay discs I have to enable it or else the PS5 might show the video all wrong. I have one disc in particular, Top Gun Maverick, which looks absolutely horrendous if I haven't enabled HDR on the PS5 before playing it. All the colors are muted and very yellowish, and it looks just horrible. When I enable HDR, it looks pretty ok, but this is a hassle. It also doesn't help that the PS5 player appears to re-send the TV a signal that "hey, we want HDR now" at the beginning of each "chapter" of the BluRay, which causes and annoying "HDR signal" message on the TV to appear. I have not found a way to disable this message.
And do you know what's the worst thing about this? The fact that I literally cannot see any difference between the 4k BluRay videos and any 1080p BluRay video, in terms of picture quality, clarity or crispness. It all looks the same to me, no matter how far or close I look at the TV.
Because of all this hassle, I am never again purchasing a 4k BluRay. The old 1080p BluRays are hassle-free, and equally good in picture quality (if not even better because they don't suffer from the technical issues with HDR.)
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