Macbook Air 2015 4k?

Hello. I read some confusing info on 4k for Macbook Air.

Macbook Air HD 6000 up to 2560 by 1600 pixels on an external display.

So can the Macbook Air 2015 be used with an external monitor 4k at @60hz? I am trying to decide between this and a pro.

I plan to use a Dell 4k. I also read this. Using 4K displays and Ultra HD TVs with Mac computers - Apple Support

I read this on another site. "You are quite correct the HD6000 does support 4K @ 60 Hz. However Apple have not enabled this functionality."

Thanks if you can help answer this.