I don't see a 4k display coming out of Apple and for more than one good reason.
First of all, 4k with anything other than the new mac pro would sacrifice way too much performance.
2nd, limited usage for 4k video- you wouldn't typically utilize the full 4k except for the final rendering.. you certainly wouldn't render 4k previews into your buffer while editing.. if the performance isn't a issue, the UI would get in the way.
3rd, 4k doesn't really sit neatly in line with OSX's retina-scaling solution; the pixel-density with 2x scaling would be too low on a 27" monitor(better suited for 21.5"), and too high at it's native resolution (even at 32"). They would need to default to some non-integer scaling solution - not something I see Apple doing.
Lastly, thunderbolt's 1-cable solution doesn't have the same appeal to the professional using a mac-pro "desktop" since they won't be unplugging their display all the time.
A 3rd-party non-thunderbolt 4k display makes much more sense to have.
A WQHD x4(retina) display would make a little more sense than a 4k display for Apple to make as it would address issues 2&3, but would also be ludicrously expensive. If they did do that, also interesting is the prospect of including additional processing power to manage the retina scaling - that way, it might appeal to people wanting to hook up their MBA or MBP for some extra eye-candy.
Like many people, a updated 27" WQHD TBD is more exciting to me.
A updated slimmer chassis, the full-laminated glass, usb3, tb2, and updated magsafe is more than enough features to justify a updated model. Highly unlikely, but a hdmi input or at least compatibility with display-port would put icing on the cake. I don't know why they haven't released this yet unless they're still having trouble with the supply-chain.