Earlier quoted context omitted.
At 4 inches, the maximum the human eye needs is 2190 ppi/dpi. http://wolfcrow.com/blog/notes-by-dr-optoglass-the-resolutio...
The link seems to confirm the 400+ range is getting useless fast for anyone who isn't a baby holding the screen to their face.
A baby's vision is around 20/800 [1] (extremely poor).
[1] http://optometry.osu.edu/research/VisualPerceptionLaboratory...