Thanks Ron, also this from apple:
Use 4K displays and Ultra HD TVs with your Mac
Learn which Mac computers can use 4K displays and Ultra HD TVs, and how to set up and adjust them.
Supported displays and configurations
You can use 4K displays and Ultra HD TVs with these Mac computers (special adapters might be required):
MacBook Pro (Retina, Late 2013 and later)
Mac Pro (Late 2013)
iMac (Retina 5K, 27-inch, Late 2014 and later)
iMac (Retina 4K, 21.5-inch, Late 2015)
iMac (27-inch, Late 2013 and later)
iMac (21.5-inch, Late 2015)
Mac mini (Late 2014)
MacBook Air (Early 2015)
MacBook (Retina, 12-inch, Early 2015)