Yeah, you should get a displayport cable and a mini Displayport to Displayport adapter. You could use the built-in HDMI port, but the display would get a slower framerate. The current version of HDMI is limited to 30 hertz for 4K while Displayport can handle 60 hertz at 4K, which is a lot...
I've been wanting an external graphics card for my 13" MacBook Pro for a while, but I don't want to spend much more than $25 on the adapter. Is it possible to use a USB 3 riser like this http://ow.ly/3xFI4o? I realize this isn’t the original intention of that adapter, but it uses a USB 3 cable...