QuadHD screens have hardly become the norm since first appearing on last year's premium handsets, however Sony has decided that even 500ppi+ screen resolutions aren't good enough and have equipped the Xperia Z5 Premium with a full 4K display. That equates to around 800ppi.
Whilst its a mind boggling achievement I wonder whether its actually of use for anything other than bragging rights. After all the G3's 5.5" QuadHD screen viewed against a 1080p screen didn't really offer the order of magnitude improvement that the numbers would suggest.
Don't under-estimate the value of bragging rights, especially to Sony, who have long struggled to gain recognition for some very good premium phones. If the flashy screen is enough to encourage US networks and discerning buyers to give the phone credibility which then reflects well on the more standard members of the Z5 family (the Z5 and Z5 Compact) the Premium won't necessarily have to sell all that well.
Which as just as well, because at £699 it almost certainly won't.