It’s usually done when there some higher res footage in the mix - for example The Revenant was mostly shot on standard Arri Alexa at 3.4K but some parts were shot on the Alexa 65 at 6.5K, so the film was finished at 4K with the 3K parts upscaled and the 6K downscaled.

But it would actually make sense that the last two prequels would be finished as 2K (2048x858), because that’s the digital cinema projection standard. And with the amount of green-screen used, there were many shots, where the live-action footage would be scaled down inside the frame when put into an extended digital environment. Not to mention the many many shots, which were purely CGI, so if they did finish the movies at 2K, there could be a lot of shots, which would be true 2K resolution and not just upscales.

As for 4K, there are many movies being released on UHD BD, which had a 2K DI, so they are 100% upscaled from 2K, but they still can look better than regular BD, because of HDR grading. Plus, if they were so inclined, they could probably re-render the CGI bits at 4K.

Plus, up until about a year ago, even movies shot on 35mm had their digital effects rendered at 2K, which means that not only the fully CGI scenes, but any live action containing even a little bit of digital tinkering, are stuck at 2K, so for example the LOTR trilogy is in the exact same boat as far as 4K, as the Star Wars prequels.