Thunderbolt vs DisplayPort I have a Mid 2015 15" Retina MBP and two external 4k monitors. Both monitors work when plugged into the HDMI port, but I can't get the second monitor to work via the display port, and need clarity.

I want to connect two 4k monitors to the laptop. I would like to connect one with the HDMI cable and the other with either mini display port or Thunderbolt.

I was under the impression that Thunderbolt could double as mini display port, so I have tried a few different Thunderbolt cables. At least one of these cables is known-good (it works for transferring data when the machine is in target disk mode).

So am I wrong in thinking I should be able to use a Thunderbolt cable as a mini display port cable? (A coworker suggested there might even be a firmware update needed, but I can't corroborate that via Google.) What am I missing?
Thunderbolt and DisplayPort are not compatible. They only share one port. Thunderbolt uses active transmission (there's a chip in the cable), DisplayPort uses a simple copper cable. You should be fine with using a DisplayPort cable.