Only Macs with Thunderbolt 2 support 4K.
And only the 15" late-2013 rMBP and nMP support 4K at 60 Hz, officially.
H-Processors: 3840 x 2160 @ 60Hz (Ultra-HD) U-Processors: 3200 x 2000 @ 60 Hz, 3840x2160@30Hz Y-Processors: 2560 x 1600 @ 60 Hz
It's important to remember that, especially with integrated graphics that share system memory, a 4k image will take a lot of memory to process, not to mention the 28gigabit data throughput just to keep the image in sync on the screen.
Even when you drop the refresh rate to 30fps, you still need 14gigabit throughput, which is higher than Thunderbolt's 10gbps - thus the requirement for 20gbit Thunderbolt 2 devices.