So with all the technical details out of the way in part 1, let's have a look at where to get 4K content and whether it's worth the trouble and the expense. Where I've found 4K content:
Youtube. There's some 4K content on Youtube. A lot of it is regular Youtube stuff, which doesn't really need the extra resolution. However, there are some really nice demos of landscapes and the like. Note that the Apple TV won't play Youtube 4K content in 4K due to disagreements about video codecs between Apple and Google.
Amazon Prime Video. A good number of their originals are in 4K and sometimes HDR, but as far as I can tell not in Dolby Vision. Also, no Dolby Atmos. I haven't seen any 4K catalog content on Amazon Prime Video.
Netflix. It looks like most of Netfilx' original content is in 4K these days, and often 4K Dolby Vision and Dolby Atmos. Recently, I've encountered a few older catalog titles that were in 4K, but not Dolby Atmos or HDR (Ghostbusters, Christine, District 9). You do have to pay a few euros/dollars a month extra for 4K with Netflix, and you can only tell that a title is 4K, Dolby Vision and/or Dolby Atmos on a device that supports playing back those formats, such as the Apple TV.
Apple iTunes Store. When you buy movies from Apple, you can download the HD versions but if the movie is available in 4K—which doesn't cost extra—you can only stream it in 4K, not download it. Apple supports Dolby Vision and Dolby Atmos, but those aren't available for every movie. Also, in a few cases the UHD blu-ray has Dolby Atmos, but Apple's version doesn't. Movies on the Apple iTunes Store are relatively cheap: most are 10 euros here in the Netherlands. And they have deals every week. I've set up a wish list on iTunes on my computer so I can quickly check for deals and 4K upgrades of existing titles.
UHD blu-ray. I still like to have physical copies of my favorite movies, and with UHD blu-ray you know you have the best possible video and audio quality.
When everything comes together, 4K content looks amazing. However... there's no room for error. You see all the limitations of the source material and how it was handled. For this reason, I always read the 4K UHD blu-ray reviews on blu-ray.com to make sure that a title looks and sounds good before I buy it.
One of the oldest movies I have in 4K is 2001: A Space Odyssey. On the one hand, this movie was shot on 70 mm, so it should be sharper than movies shot on regular 35 mm film. But, it's also half a century old. Turns out it's not quite as sharp as I'd expected: the upgrade over the regular blu-ray is relatively modest. Also, there's a bit of grain, especially in the space sequences. Not a huge amount, but a little more than I expected. The scenes under colored light benefit from the better color reproduction. The disc contains a 5.1 version of the original audio mix and a new mix that is very close to the original intent, which means some left/right separation, but very little real surround action.
Interestingly, a movie from the 1970s like Close Encounters of the Third Kind has scenes that are quite sharp, but the movie is also quite grainy, with a layer of really fine grain. Ghostbusters—the 1984 version, of course—which I saw in 4K on Netflix, is also sharp, but shockingly grainy.
Many people actually like grain, citing a "filmic look". I can handle a little grain, but I really don't like it, especially when it gets bad enough to be distracting. In my experience, when blu-ray.com reviewers mention grain in any way, that means the amount of grain reaches distracting levels.
Film actually has better color reproduction and dynamic range than HD, so in many cases, the 4K transfers of movies shot on film have more intense colors and some level of HDR, compared to the HD transfer. However, a lot depends on the lighting.
Obviously, before 2000 or so, all movies were shot on film. And a good number of movies are still shot on film. Around 2000, the digital era started, but cameras (background: here, here and here) as well as digital intermediates weren't immediately 4K. And many movies are still finished using a 2K digital intermediate. Sigh.
Some examples of movies that make good use of the higher resolution, wider color and/or high dynamic range:
The Revenant was shot in the western wilderness pretty much entirely using ambient light. It's very sharp, but the colors are mostly muted. They originally wanted to shoot the movie on film, but it turned out that that wouldn't work, so they used digital Arri Alexa 3.4K and 6.5K cameras.
Blade Runner 2049, set in a noir future, also looks nice and sharp and has spectacular colors in some scenes.
Passengers, which is set on a spaceship that's taking 90 years to reach another star, was problematic for me for a reason that will be obvious if you've seen it, but it does look very good, and especially the non-effects scenes are very sharp.
Mortal Engines has steam punk roots, with cities roaming the landscape on wheels. The visuals are really stunning but unfortunately you don't get too much time to absorb them.
The Martian is a really entertaining film. The mars outdoors shots, especially the space suits, look really good. But apparently the movie was finished using a 2K digital intermediate, so it could have been sharper.
Pacific Rim has crazy colors, as well as crazy robots-on-giant-monster mayhem. Also finished in 2K, but due to darkness, rain and the fast action, we weren't going to enjoy the full 4K sharpness anyway. One thing I like: this movie is in the 1:1.85 aspect ratio, so it fills the entire screen with no black bars.
Dunkirk, a movie about what happened in Dunkirk during World War II, was filmed in 70 mm and finished on film, a rarity for a 2017 movie. It's quite sharp with minimal, if any, grain. The colors are a bit off, though. And I didn't like the audio mix: it's too sparse. Stuff that's on screen, such as a breeze moving the grass on the dunes, isn't in the audio mix. I also didn't care for the story and the strange way the timelines of the subplots are handled.
Oblivion, a science fiction movie with Tom Cruise, is a bit sharper in 4K than in HD, but with the 2K digital intermediate and the somewhat muted colors, the upgrade isn't super obvious. It does make really good use of the surround and height speakers, though.
A Quiet Place shows a future where blind creatures kill anyone who makes a sound. As such, large parts of the movie are very quiet. Very good Dolby Atmos mix.
Edge of Tomorrow, where Tom Cruise finds himself on the battlefield fighting an alien invasion and keeps waking up on the same day every day after he's been killed. Also isn't super sharp and moves too fast anyway, but it has a very good audio mix, even in in 5.1.
The Maze Runner is a dystopian "young adult" story, and one where both the protagonist and the viewer have no idea what's happening until almost the end of the movie. But it works, in my opinion. There's some nice color and sharpness is fairly good, but I especially like the audio mix, which is in 7.1.
Alien, The Revenant and Blade Runner 2049 have nice audio mixes, in 5.1, 7.1 and Dolby Atmos, respectively.
So far, I've only encountered one 4K release that disappointed me across the board: Star Wars The Last Jedi.
The problems start immediately after the opening crawl, as we get to see some huge space ships. The issue: space isn't black here, it's dark gray. In other scenes, black levels are ok, though. Sharpness and colors are a bit stronger than on blu-ray, but I didn't notice any HDR use. The Dolby mix didn't use the height channels very much, if at all. But the worst thing is the grain: there is really no excuse for this level of grain in 2017. Both The Force Awakens and The Last Jedi were shot on film, but the former is not nearly as grainy as the latter, although it's a bit hard to compare without a 4K UDH blu-ray release of The Force Awakens.
These are some movies that I'd love to see released in 4K Dolby Vision because of their color use, even though they're finished using a 2K digital intermediate:
I'm interested to see Gravity and Tomorrowland for their HDR use. Gravity was finished in 2K, Tomorrowland in 4K. They used Tomorrowland as a showcase for Dolby Vision in a new movie theater, and the transitions between dark and light were very impressive.
Last but not least: Star Wars The Force Awakens. There's lots of eye candy in this movie and enough time to admire it, so bring on the 4K.
Amazon Prime and Netflix release many of their own TV shows in 4K, usually also in HDR or Dolby Vision. (And sometimes Dolby Atmos.) Presumably, the same will be true for new content on Disney+ and Apple TV+ later this year. I believe producing in 4K doesn't really cost any extra for shows or movies that don't contain CGI special effects. I'm not a huge fan of these TV shows, so maybe I've missed some excellent use of 4K. But what I've seen is anything ranging from a negligible to a modest uptick in sharpness. There's some extra color here or there, but again, nothing to get too excited about.
First of all, HD is just fine. If you have better uses for your money, don't worry about 4K right now. I'm sure in another few years, everything will be 4K so you'll get it at some point when you need to replace your TV anyway. In the meantime, many UHD blu-ray movie releases also contain a regular blu-ray, so if you buy movies on blu-ray, consider getting the blu-ray/UHD combo, so you'll have some content when the time comes to upgrade.
Of course if money is no object: replace all your stuff. 4K is mature enough to do it now rather than wait.
If you're on the fence: I'd say, have a look at what kind of content you like to watch. Comedies and dramas are of course sharper in 4K, but that doesn't add all that much to the enjoyment of those genres. For instance, I love Groundhog Day and Office Space and I'm glad I have them in 4K. But I wouldn't replace my TV just to see those movies in 4K.
It's a different story for many science fiction and action movies and epics. Those often have the kind of visuals that can benefit from the higher 4K resolution, more colors and more dynamic range, and of course the kind of action that gives surround speakers a workout. Today's blockbusters have huge budgets, with a good amount of that ending up on the screen. So if these are the kinds of movies you like, a 4K TV and a Dolby Atmos setup can be money well-spent.