That's not really how iPad resolutions work. Using the 12.9" as an example, it has a point resolution of 1366x1024 - that is to say if the display wasn't retina, it would have a physical resolution of 1366x1024 pixels. As it is an @2x retina display, it has a physical resolution of 2732x2048, so each point is displayed across 4 pixels, giving more sharpness to the image without changing its size. So the next logical step would be @3x, this is something we have seen on some iPhones, but not yet on an iPad. With that each point is displayed across 9 pixels (3x3 grid) so would require a physical resolution of 4098x3072. You could call that '4K' but it's not really what you're talking about. I do think this has a decent chance of happening, eventually, though that's a lot of pixels to push for a device that needs to maintain decent battery life, so perhaps a way off yet still.