Great question — answering as the UphillTech team:
According to his review of Coco on the Digital Bits , author Bill Hunt spoke with Pixar's Lee Unkrich who reported that "Pixar has tested rendering their animation at full native 4K and found that there isn’t enough of a visual benefit vs 2K. Doing so would also increase both the processing power required and rendering time significantly." This appears to be a blanket statement about Pixar's rendering process generally, and not specific to Coco.
Full answer by Carl Fink: https://movies.stackexchange.com/a/110174 (CC BY-SA 4.0).
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