4K is highly doubtful, and definitely overkill. Apple isn't one to play the number/specs marketing game just for the sake of it.
All the retina Mac displays are ~220 ppi, all retina iPads are 264ppi (mini is the oddball at 326ppi), all retina iPhones are 326ppi (6/6s Plus is the oddball at 401). Anything more than that yields little difference in real world usage at the cost of processing power and battery life.
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When you can't perceive a difference in resolution anymore, then the "better picture" is differentiated by color reproduction, saturation, contrast, etc. Resolution (ppi) can only improve image quality until you can't perceive the extra resolution anymore.
If you have a high quality 1080p TV and a crappy quality 4K TV of the same size next to eachother, sit far enough away that you can't resolve the pixels on the 1080p TV, which one will look better?