I read they actually scanned a theatrical release 35mm print for this remaster. That's such a huge no-no. Some tonemapped screenshots are floating around the web and the results aren't great. Compared to the 1080p blu-ray the color grading on this new master looks bad and there is even less fine image detail and it seems like there is less dynamic range on the UHD Blu-ray compared to the 1080p Blu-ray.
Sounds like it is an improvement over either previous Blu-rays though -
Previously on Pan's Labyrinth: Those who have bought this film on earlier formats (DVD, HD DVD, and/or Blu-ray) likely know that Warner Bros.' 2007 Blu-ray (https://www.blu-ray.com/movies/Pans-Labyrinth-Blu-ray/611/#Review) and Criterion's 2016 Blu-ray (https://www.blu-ray.com/movies/Pans-Labyrinth-Blu-ray/156565/#Review) offered fundamentally different 1080p images. The former was a decent early-format presentation unfortunately slathered in digital noise reduction, while the latter contained a few director-approved color tweaks and more prominent black levels...but was also slightly hampered by a few mild compression-related issues. Pound for pound, Criterion's disc still easily came out on top.
The quickest summary of Warner Bros.' new 4K disc is that it looks like a more ideal version of their previous Blu-ray. The film was finished at a 2K digital intermediate (likely because of its often lower-res VFX, whose seams now show a bit more), so don't expect any kind of quantum leap forward in regards to fine detail and texture. Still, this new 2160p transfer clearly serves up a more crisp and satisfying presentation overall, thanks as much to its lack of DNR as the format's higher resolution. Outdoor daytime scenes obviously fare the best, with dense foliage and close-ups revealing subtle details not nearly as noticeable in 1080p. Other moments are absolutely bathed in black, but even these are afforded a boost by the slightly more refined shadow detail. Compression artifacts are also not an issue, especially due to the disc's generous bit rate which often peaks at or above 95Mbps. Overall, this 4K presentation marks a solid step up from both earlier Blu-rays, but for entirely separate reasons. And while those director-approved color tweaks from the Criterion disc have not been fully applied to this transfer, the format's HDR grading does predictably boost depth and saturation in key scenes; that's a good thing, as having both would have likely been overkill.
As mentioned earlier, the included Blu-ray is the same as Warner Bros.' 2007 release, for better or worse. While its 1080p transfer is serviceable at first glance, I am a lot more critical of certain flaws -- namely, the excessive noise reduction -- and thus the "Video" rating above reflects a lower score than what it originally got in 2007.
https://www.blu-ray.com/movies/Pans-Labyrinth-4K-Blu-ray/241970/#Review (https://www.blu-ray.com/movies/Pans-Labyrinth-4K-Blu-ray/241970/#Review)