I think you're only going to find 4k tvs doing 120hz(in 1080p), because it requires the same bandwidth as 30hz 2160p (which has 4x the pixels; 4x30hz = 120hz). Otherwise tv makers aren't going to waste money on a slightly faster interface.
BTW you won't see 2160p at 60hz until hdmi 2.0 becomes standard, and their aren't even any plans for an hdmi that'll do 2160p at 120hz. You need display port for that, which means monitor, which means 6x the price.
There is a difference between 120hz input and 120hz interpolated. TVs take a 30hz or 24hz input and smooth it to display 120hz motion. Which while it generally looks good, will add a frame of lag. I'm not sure if they smooth 60hz signals or not. When it comes to TVs only 4k tvs will accept 120hz input(and only in 1080p mode).