I guess it's good to have a cable with the bandwidth to push 120Hz on a 4K monitor, but there's no reason to drive a 4K monitor at 120Hz if you have a monitor with an adaptive sync technology such as G-sync. Freesync monitors are also supposed to become available in the near future. Acer will be releasing a 4K G-sync monitor next year.
If you think of it 4K is the equivalent of 4 1080p screens. For 4K, you'll still need 3 or 4 980s (if initial specs are correct) to drive the most demanding games on ultra at frame rates consistently above 60 and a lot of times you won't be anywhere near the 120fps mark. In 5-10 years, it will be a different story.
For now, I think monitor manufacturers should focus on adaptive-sync technologies as they really make a huge difference in performance and provide more value for more people.