A pixel density of 734 pixels per inch is pretty impressive, and Chinese manufacturer Everdisplay has made such a display in a 6-inch format said to be slated for use in VR devices. A display like that would look might sweet on a smartphone too - and Sharp is working on a 5.5 inch 3840x2160 display also.
PhoneArenaAlthough a PPI density of 734 is definitely high-definition by any modern standard, bear in mind that this doesn't break any records as far as prototype displays go. Earlier this year, Sharp announced a 5.5-inch IGZO display with a 4K resolution and a PPI of 806. Furthermore, Samsung recently announced plans to manufacture a display with a mind-boggling 11K resolution, one that would feature the unthinkable PPI of 2250.