Yes, the 2015 MacBook Air supports 4K displays at 60Hz

Andrew Cunningham, Ars Technica:

When the new MacBook Pros and Airs were announced, we reported that both systems were capable of driving 4K displays at a refresh rate of 60Hz. In our head-to-head comparison of the two systems, we did it again. But in the last few days a number of readers have pointed out that Apple’s tech specs page for the new MacBook Airs claims a maximum resolution of 2560×1600.

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.