Been trying to track down the answer to this myself. Would like to run a 4K monitor in future and Intel HD Graphics 5000 should be able to manage that according to what Intel's been saying about them (I guess we shall see). Haven't been able to track down much on the chip in the new Air, but assuming Apple have fitted the new Redwood Ridge chips, it should be DisplayPort 1.2 compliant. Given they were going for battery life on these new models, I'd say that's likely (the new chips use less power).
I believe the real problem with displaying 4k at the moment is that MacOS can't do it. I did find a report of a guy outputting a 4k signal at 30hz from his Macbook Pro, but he was only able to do so using Windows+Boot Camp. When he tried it under OSX, the option was unavailable. I believe the new Mavericks version is supposed to correct this issue. If anyone's running the developer version on the new Air and also has a 4k display, I'd be very interested to hear if it works (somehow I doubt that's going to happen lol)...
Incidentally, I read something in Anandtech's Thunderbolt 2 article that suggested that when it came to Redwood Ridge, the issue with high res support is that there's no receiver chips to match with the chip in the computer. I thought that implied (slightly) that these new chips may be able to run with the new 20Gbps bandwidth (after all, it's just the merging of the two existing channels into one). Might just be wishful thinking on my part though...
