Why won't my late 2014 Mac mini output 4k video?

I have a Samsung U28D590 plugged into a Late 2014 Mac mini (with Iris video graphics) via Thunderbolt 2 to DisplayPort.

In the display options it does not give the option for 3840x2160, even after holding the option key while clicking on "Scaled"

Both the monitor and Mac mini support this resolution according to their documentation, so I wonder if I need to install some extra drivers or something like that.