There's a rumor going around that Apple might be considering going to @4x -- a "Super Retina" for a future iPhone display. Apple went to @2x, from 320x480 at 163ppi to 640x960 at 326ppi for the iPhone 4 Retina display, and lengthened it to 640x1136 at the same 326ppi for the iPhone 5's 4-inch widescreen display. These days, however, Android phones are shipping with 1080x1920 (1080p) displays at well over 400ppi.
So, to leap ahead again, and to keep things just as easy on the apps as the last big density increase, in some theoretical universe Apple could just double again to @4X, right?
That's how the various 5-inch iPhone options look when you compare them both in terms of pixel count (top) and physical scale (bottom), with an iOS standard touch-target grid overlay (44px). From left to right, the iPhone 5 (black), theoretical iPhone @3x (purple), theoretical iPhone @4x (red), theoretical iPhone with iPad display @2x (blue), with the HTC Droid DNA 1080p (green) thrown in for good measure.
Yes, HTC has made 1080p (1920x1080) phones at 440ppi, which while positively pornographic, show what's already out and on the market. If Apple decides it wants to exceed that the way the original Retina display exceeded other panels at the time, they'd have to skip 3x and go straight to 4x -- 2272x1280 at 522ppi.
It would require even more graphics processing power, more LED light, and more battery, and it would be absolutely batshit insane, but hey, we're considering all options here.
Going to 5-inches @4x probably isn't something within the current realms of technology, economics, or reason, however.
Going to to @4x with a 4-inch display, to put it just as bluntly, is even more batshit insane. Until humans grow hawk eyes, there's just no need. If an when Apple goes to 5-inches, maybe @3x will make sense, or maybe a more painful switch to the 1080p standard. Maybe there's even something in the Apple labs that does 2x Retina in a very different way than what's being reported right now...
Anyone actually want to see Super Retina display any time soon?