There have been some 4K TVs shown off over the last couple of years or so, but that 4K term was never the best way to describe it. It just didn't have a good fit, especially since the resolution varied on some sets. Recently, the Consumer Electronics Association (CEA) announced the official name for 4K TVs as Ultra High Definition, with the International Telecommunications Union (ITU) declaring future 4K and 8K TVs will be marketed as Ultra High Definition Televisions. All UHDTVs must have a panel resolution of at least 3840x2160, one or more digital inputs capable of delivering a 4K signal, and be able to display 4K content natively and not just upconvert lower resolution content. It's rather nice to have an official name for these 4K screens, with Ultra High Definition being a logical step since the resolutions are four times greater than current HDTVs.
The only downside, right now, is UHDTVs are not the most readily available or affordable, and the lack of a ton of 4K content. YouTube has some 4K resolution videos available, but there isn't much else at the moment. UHDTVs are expected to have a much larger role at CES 2013 in January, and hopefully then we'll learn when to expect more 4K content.