I realized that I needed to clarify what I meant by 4K. I do not consider either the current 2048x1536 or even the current iPad Pro's 2732 x 2048 anywhere
near 4K. 4K would, for me, require a minimum horizontal resolution of 3840 pixels - for the iPad's 4:3 ratio, that means a final resolution of 4096 x 3072.
What this essentially means is that you're going from 3 megapixels to 12 megapixels - this is a huge jump in resolution and will necessarily require an enormous amount of computing power. Even as you say,
@VanillaCracker, 4K resolution does stress current mainstream graphics - in a portable device, I don't think current graphics hardware is really up to the task unless you restrict viewing to text or other non computationally intensive output.
On the other hand, I think "2K" (by which I assume around 2560 x 1600, or otherwise similar to the iPad Pro's 2732 x 2048) is actually very reasonable. I actually think a 4K screen on the 12.9" iPad Pro would be great, but of course, it should not come at the cost of overall performance and battery life.