"With OS X Yosemite 10.10.3 and later, most SST 4K (3840 x 2160) displays are supported at 60Hz on these Mac computers:
MacBook Air (Early 2015) and later
MacBook Pro (Retina, 13-inch, Early 2015) and later
MacBook Pro (Retina, 15-inch, Mid 2014) and later"
https://support.apple.com/en-us/HT206587Earlier years will only do 30Hz, which is fully usable for development/office work.