Most likely with the release of the new MacPro around September. I'd expect a 4k panel based on the Mavericks desktop resolution.
I think that they'll split their display lines. The new MBA only supports up to 2500px, which perfectly fits the current Thunderbolt Display. The new Mac Pro will support 4K and I expect that the next 15" MBP will do as well.
So I doubt that they'll dump the Thunderbolt Display and leave the MBAs behind with no external display solution.
My guess is that we'll see an updated TD in the new iMac's design, using the same display technology. And a new Cinema Display with 4K for the pros. At a ridiculous price, though.