It's worth noting that at 120hz you need very good pixel response time to help you visibly see the difference, a lot of these Korean panels that "overclock" to 120hz look rubbish because the pixel response times (these are IPS?) is pretty bad.
120hz on an LED TN panel with a 1-2ms pixel response time brings back that old CRT responsiveness feel that LCDs have missed for a long time, and it's well worth the premium for gamers.
Unfortunately getting large panels like 4K running at 120hz just isn't feasible, it terms of data per second it's getting into the realm of pushing the boundries on our technology, we don't have any cable formats to do 4k@120hz to my knowledge, the data processing inside the monitor itself would be extremely expensive (features like scaling that cost more performance as refresh rate and pixel count goes up), and lastly the actual GPU to power 4k@120hz would simply be ridiculous.
4k@120hz would require 8x more GPU power than 1080p@60hz, so you're talking about rigs where you take 4x dual GPU cards to really get that kind of performance, ignoring crossfire/SLI overheads and scaling issues etc, it's just not going to happen. Maybe 3-4 Generations from now we can do it reasonably, but all these technical barriers make it extremely unattractive for manufacturers.
Don't get me wrong I have 120hz TN and love it, and have a 60hz IPS @ 2560x1600 and love that, but at this stage you pick one or the other, high res or fast refresh rate, I mix and match to get the best of both worlds.