I have a Macbook Air, early 2014. How do i get 3840x2160 resolution output?
That Macbook Air is only capable of 2560 x 1600 maximum resolution, according to Everymac


  Although Apple only reports that this model can support a single external display up to 2560x1600, Intel reports that this model can simultaneously support two external displays up to 2560x1600 "daisy chained" via Thunderbolt.


There's a possibility you are limited to 1080p by the HDMI cable. make sure it's HDMI 1.4 capable.