'4K' is marketing bullshit either way; but can be one of several different flavors of marketing bullshit, depending on who is talking about it.
DCI 4K is 4096 x 2160, 17:9, and backed by the movie studios as a digital theater projection format. This makes it pretty well established in terms of cameras and workflow tools; but the only playback devices are wildly expensive projectors with DRM that makes HDCP look like a Pirate Party darling.
The TV guys back 'UHD 4K', which is 3840 x 2160, 16:9, and appears to be the resolution of everything you can actually buy, or seem likely to be able to in the immediate future.
There is also HXGA, 4096x3072, 4:3, which appears to be entirely theoretical.
The main kicker in the cheap seats, right now, is that HDMI 1.4 does not have the data rate to drive a UHD display much above 30 FPS. Displayport does (but the cheap TVs don't have it) and HDMI 2.0 apparently will; but is essentially unreleased at present.