With the release of OS X 10.10.3 last Wednesday, Apple has expanded support for high-resolution 4K and even 5K external displays



From MacRumors:

Most notably, OS X 10.10.3 enables the Retina 5K iMac and 2013 Mac Pro to drive Dell's UP2715K 27-inch 5K display released late last year. The display requires more bandwidth than is currently supported over a current single DisplayPort/Thunderbolt cable, so it uses a dual-cable solution taking up two ports on the user's machine.

  OS X 10.10.3 Now Supports Dell's Dual-Cable 5K Monitor on Retina iMac and Mac Pro