Eizo's ColorEdge CG276 has already been on the market for quite a while, but a new update will allow it to support 4K signals. While the previous models wouldn't, any model produced as of now will be able to support 4K signals as well as 2K signals.
The monitor itself doesn't have a 4K panel; in fact, its resolution is 'just' 2560 x 1440. What the unit does is scale down the UHD (3840 x 2160) or 4K (4096 x 2160) resolution to the panel's resolution. Granted, at these resolutions the refresh rate will only be 30 Hz.
The reason why Eizo has decided to update the monitor to support this is because in the film industry 4K is becoming a standard at an ever-increasing rate. While televisions are slowly gaining 4K support, computer monitors are not. The WQHD resolution (2560 x 1440) is still far away from becoming a standard, so obviously, it'll be a long wait until we see 4K computer monitors.
Eizo's ColorEdge GC276 has already been on the market since November 2012, and it now has a street price of about $2,300.