Can Macbook Air 2015 early model's thunderbolt 2 support 4k 60hz ?

Apple's Technical Specifications say that the Early 2015 MacBook Airs support UHD 4K resolution (3840 x 2160 pixels), but do not promise support for a 60 Hz refresh rate.

MacBook Air (11-inch, Early 2015) - Technical Specifications

MacBook Air (13-inch, Early 2015) - Technical Specifications

• Dual display and video mirroring: Simultaneously supports full native resolution on the built-in display and up to 3840 by 2160 pixels on an external display, both at millions of colors

The Thunderbolt 2 port has a Mini DisplayPort connector, and can double as a Mini DisplayPort. So the most direct connection would be

• Mini DIsplayPort to DisplayPort
• Mini DisplayPort to Mini DisplayPort

but you could also use a Mini DisplayPort to HDMI adapter or adapter cable. I'd recommend looking for adapters, and adapter cables, that are advertised to support 4K @ 60 Hz. Even if it turns out that the computer will hold the refresh rate back to less, at least you can avoid buying a cable that you know would be a bottleneck.

MacTracker indicates that the Early 2015 MacBook Airs support "Up to 3840 x 2160 pixels at 60 Hz."

According to an ArsTechnica article, when your MacBook Air first came out, Apple's Technical Specifications said that it could only support 2560 x 1440 pixel resolution. After ArsTechnica did some testing, Apple updated their Technical Specifications page to confirm 3840 x 2160 pixels support (without a refresh rate figure).

Note that the 2015 MacBook Airs may have performance issues when driving a 4K display. See the ArsTechnica article for more details.

ArsTechnica (March 2015) – Yes, the 2015 MacBook Air supports 4K displays at 60Hz