MacBook Pro Retina 15 connecting a 4k monitor
An Apple store employee told me they no longer sell Mini-DP to Mini-DP cables because Thunderbolt cables can serve the same purpose, but this answer suggests she might have been lying.

I've seen a firmware revision reported for a Thunderbolt cable, which fits with the claim that a Thunderbolt cable includes a chip.

I've had good luck with DisplayPort to Mini-DP cables. If your monitor has a full size DisplayPort connector, connecting a cable to that might be your best bet. Cables for that purpose are easy to find on Amazon.

This answer confirms my experience, that a Mac HDMI port can't do UHD at 60 Hz.