Originally Posted by SolipsismX
So you're saying we should keep pixel densities at a point where we can still resolve individual pixels? Brilliant¡.
Whether that was the intent of his comment or not, the "Retina" display already has more pixels than the eye can individually detect. To cram 4K density into a 13" screen does not make any sense in a consumer device, or even a pro device. The whole movement toward 4K, aside from a way to continue to expand the home entertainment business, is driven primarily by the massive flat screen sizes that are becoming common place in average Americans living rooms thanks to plummeting prices. 1080p looks amazing on a 32" flat screen even from a few feet away, at which close distance few people make a habit of watching TV. But even the average viewing distance of 8 feet away, a 50" screen begins to break down, to say nothing of an 80" screen. In 5 years, 120" screens will likely not be uncommon in many home theaters.
So considering a 13" device like an iPad would not only be taxed to supply the raw processing muscle, not to mention a power supply capable of driving that many pixels, with little or no real world payoff in the ability to benefit from the enhanced resolution at that size, I have a hard time imagining Apple would do this as a first step into 4K. Honestly, who would be able to take advantage of this? Certainly not the iPad's typical users. Apple has been notorious for lacking in their support for professional photographers on the iPad, one of the few who might benefit from such pixel depth, so who would they be doing this for (or have they turned over a new leaf)? Doctors who want to look at X-rays and cat scans? Even those arguably don't need higher resolution than is already offered on the iPad. And aside from movies, the iPad is primarily a scalable device anyway, relying on software to resolve pixels through sophisticed algorithms at larger than native sizes. That won't change thanks to added pixel density, any more than it did from the original iPad to the Air. People will still mostly be looking at images in something other than their native resolution.
Unless I'm missing something, this seems like a totally unrealistic rumor to me. If Apple were going 4K, it seems likely they would make that jump first where it would count the most -- the desktop, or Apple TV, skipping Retina altogether, then merely to consolidate compatibility across platforms, add it to the iPad when cost and power issues were resolved.