PPI is different than resolution. So at 5.5in the PPI is about the same as the PPI on some 4K TVs but its not 4K. Its kind of how 1080p on the iPhone looks amazing but 1080p on a 70in tv viewed at around the same distance as an iPhone looks blurry because you can see the pixels when you are close enough. But 4K has 4 pixels for every 1 compared to 1080p giving you that higher resolution, clarity and definition.
Now, even if the iPhone had 4k the screen it would not seem that much better because of Retina. The philosophy of retina is having a PPI so tight that they human eye can not tell the difference between pixels at the normal viewing distance. So adding even more pixels when your eye cant tell the difference starts to be a moot point. But with larger display you start to add definition. and that's were 4k really shines.