I'm a little confused as to how this works. It appears that the movies are still 1080p, but are "optimized" for 4K somehow.
"The Blu-ray “Mastered in 4K” collection is optimized so that 4K Ultra HD TV owners can take full advantage of the new 4K upscaling technology, delivering an outstanding near-4K experience. “Mastered in 4K” Blu-rays also work with all 1080p HDTVs for spectacular high definition picture and sound. "
Note "4K upscaling"