While previously Apple only officially supported certain Multi-Stream Transport (MST) displays at a refresh rate of 60Hz, it now says that “most single-stream 4K (3840×2160) displays” are officially supported at 60Hz as well following the recent OS X Yosemite v10.10.3 update. That should mean support for a lot more inexpensive 4K displays that don’t include DisplayPort’s Multi-Stream Transport feature.
A full list of Macs that will support Single-Stream (SST) displays with a 60Hz refresh rate include:
The best 4K & 5K displays for Mac
-MacBook Pro (Retina, 13-inch, Early 2015) -MacBook Pro (Retina, 15-inch, Mid 2014) -Mac Pro (Late 2013) -iMac (27-inch, Late 2013 and later) -Mac mini (Late 2014) -MacBook Air (Early 2015) -MacBook (Retina, 12-inch, Early 2015)
SST displays running at a resolution of 4096×2160, however, are only supported at 60Hz on the Mac Pro (Late 2013) and iMac (Retina 5k and 27-inch, Late 2014).
For Apple’s new 12-inch MacBook, the device will support displays running at a resolution of 3840×2160 at 30 Hz or 4096×2160 at 24 Hz over HDMI. Since the device only has a single USB-C port, you’ll need a USB-C Digital AV Multiport Adapter. Technically it’s possible Apple could support monitors hitting the market that use HDMI 2.0 at 60 Hz, but you’ll of course still need the appropriate USB-C adapter.
If you want 60Hz on the new 12-inch MacBook, you’ll need an SST display, DisplayPort cable, and OS X 10.10.3. Although you’ll also need to get an adapter in order to go from the 12-inch MacBook’s 1 USB-C port to DisplayPort on the display— like this one.
Additionally, Apple’s update to its 4k support doc confirms that it’s officially supporting Dell’s new UP2715K 27-inch 5K display following the OS X 10.10.3 update on the Mac Pro (Late 2013) and iMac (Retina 5k and 27-inch, Late 2014).