I have a UN40HU6950 and have been through all kinds of troubleshooting, wanted to ask this here. Here's a thread on the nVIdia Forums:
https://forums.geforce.com/default/topic/792167/geforce-900-series/samsung-hu6950-hu7000-at-4k-60hz-4-4-4/1/
I have a GTX 970 with a new HDMI cable rated at 18Gbps. My TV is subsampling colors when I run it at 60hz, which is unusable for a computer monitor. Here is a screenshot of the in-TV-menu that tells me it is supposed to support it:
https://dl.dropboxusercontent.com/u/974249/UN40HU9650-picture-options.JPG
4k Test Image at 30hz: https://dl.dropboxusercontent.com/u/974249/UN40HU6950-4k-30hz-small.jpg
4k Test IMage at 60hz: https://dl.dropboxusercontent.com/u/974249/UN40HU6950-4k-60hz-small.jpg
It appears that Samsung has really misrepresented themselves.