I can't speak from experience about the GS4 - though I've read great things about the display that say SAMOLED is finally catching up to IPS LCDs in terms of color reproduction accuracy.
I will say though, as an owner of both the iPhone 5 and the HTC One - the increased PPI of the One does little to enhance the display quality. While it is somewhat noticeable, it is by no means a huge difference. I would say its less of a difference than pre-retina iPads to retina iPads and many people couldn't tell the difference there (and I think they're insane....).
Anyways, as far as display quality goes, I think the market is plateauing. Displays can get more color accurate (though the iPhone 5, HTC One and GS4 are all pretty close to max) and more power efficient, but I think it's safe to say, simply increasing the PPI from here on out will be just another marketing ploy to make one's phone seem better than the others' when the reality is no added benefit to the end user.
EDIT: As for Assault's pictures above me, I'd venture a guess to say most (if not all) won't be zooming in to see every individual pixel - ever.