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From Apples webpage:
With OS X Yosemite v10.10.3 and later, most single-stream 4K (4096x2160) displays are supported at 60Hz on the following Mac computers:

  • Mac Pro (Late 2013)
  • iMac (Retina 4K, 21.5-inch, Late 2015)
  • iMac (Retina 5K, 27-inch, Late 2014 and later)
  • MacBook Pro (Retina, 15-inch, Mid 2015) with AMD Radeon R9 M370X
These Mac computers automatically detect and support MST-enabled displays at 60 Hz:

  • MacBook Pro (Retina, 15-inch, Late 2013 and later)
  • MacBook Pro (Retina, 13-inch, Early 2015)
  • Mac Pro (Late 2013)
  • iMac (Retina 5K, 27-inch, Late 2014 and later)
  • iMac (Retina 4K, 21.5-inch, Late 2015)
 
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