The site had already tested the new notebooks using 4K 60Hz displays but when viewers noticed that Apple said it could only run 2560x1600 displays, it looked into the issue more closely.
We were fairly sure that Apple’s spec page was incorrect, but to get a definitive answer we dove into the still-nascent 4K display ecosystem to do some testing. The short answer is that yes, both the Pros and Airs now support 4K output at 60Hz using a single DisplayPort 1.2 cable. Last year’s Air connected to the same display with the same cable can manage, at best, a blurry 2560×1440.
Intel's documentation reveals that the Broadwell-U processors and their integrated GPUs support 3840x2160 at 60Hz over DisplayPort 1.2. The new MacBook Airs feature Thunderbolt 2 which explicitly supports DisplayPort 1.2 and makes using an external 4K monitor possible.
It's possible that Apple didn't list support for the higher resolution monitors as there is some performance issues when entering full screen mode or Mission Control
Given that the Air is using one of Intel’s integrated GPUs, general OS X user interface performance isn’t too bad while driving the Air’s internal display alongside the 4K display. Dropped frames are clearly visible when entering into Full Screen mode or using Mission Control, and of course you’ll never want to try playing games or doing heavy 3D work at native resolution. But things are more than smooth enough for desktop use.
More details on the new MacBook Airs can be found here.