My understanding is that this is pentile, meaning each ‘pixel’ is either R+G, or G+B; not the R+G+B pixel you’d expect. In practice this means 4k brightness detail, but only 2k colour detail.

That aside, 1080p isn’t past the threshold of imperceptibility. 4k totally is, but doubling a common res means you get all the benefits without non-integer scaling issues or changes in screen real-estate.