Macbook air 2017 isn't displayig in 4K (Samsung u28e590ds)

Hi,

I'm using a macbook air 2017 and I recently bought a Samsung U28E590DS monitor. In the About this mac > Displays menu it says my display is at 3840x2160 resolution. My monitor says the same (it also says that it's 30hz), but in Youtube and Netflix, the highest resolution I can watch videos in is 1080p.

I'm using the Belkin 4K Mini DisplayPort–HDMI-adapter and the hdmi cable included in the monitor's box.

Is there any way to solve this problem or I should return my monitor?

Thank you for the answers.