Shorter answer.
Do we agree on one official resolution?
No. Yes. Maybe. All depending on where and what context the “4K” designation is used.
On a basic technical level, “4K” is 4096 x 2160 and nothing else.
But thanks to competing manufacturers creating competing hardware for formats and media specs that are themselves competing against each other on another level, the whole meaning of “4K” has been bastardized to the point it’s no longer really a true consumer specification, less of an industry standard and even more of a conceptual headache than you can imagine.
So yes, it’s confusing. And confusing to the point that none of this is beneficial to the consumer in any way. More details below.
Longer answer.
What is 4K?
The 4K Wikipedia article basically states that “4K” is 4096 x 2160 and nothing else:
A 4K resolution, as defined by Digital Cinema Initiatives, is 4096 x 2160 pixels (256:135, approximately a 1.9:1 aspect ratio). This standard is widely respected by the film industry along with all other DCI standards.
What is UHD?
And then this explains what “UHD” is—3840 x 2160—and why the “4K” designation in this case doesn’t help anything; bold emphasis is mine:
DCI 4K should not be confused with ultra-high-definition television (UHDTV) AKA "UHD-1", which has a resolution of 3840 x 2160 (16:9, or approximately a 1.78:1 aspect ratio). Many manufacturers may advertise their products as UHD 4K, or simply 4K, when the term 4K is traditionally reserved for the cinematic, DCI resolution. This often causes great confusion among consumers.
Why isn’t 4K just called 2160p?
You ask this:
Also, on a sidenote, if 4K means 4xFullHD (2x1920 by 2x1080 => 3840x2160), shouldn't FullHD be called 2K?
You would think that. But then more clarification on where the mixup might have began from Wikipedia; again the bold emphasis is mine:
The use of width to characterize the overall resolution marks a switch from the previous generation, high definition television, which categorized media according to the vertical dimension instead, such as 720p or 1080p. Under the previous convention, a 4K UHDTV would be equivalent to 2160p.
So for 720p and 1080p that is defined by image height but “4K” is defined by its horizontal. So by old “standards” “4K” should then just be called 2160p.
What is QHD?
The switch in dimension measuring methodology pretty much opens up the door to anyone fudging “4K” to using it for defining items that are not “4K” in the defined sense. For example, here is some info from the Wikipedia entry on “QHD”; once again bold emphasis is mine:
QHD (Quad HD), also sometimes advertised as WQHD due to its widescreen shape, or 1440p, is a display resolution of 2560x1440 pixels in a 16:9 aspect ratio. It has four times as many pixels as the 720p HDTV video standard, hence the name.
So that explains that. QHD is “4K” in the sense that it has four times as many pixels as the 720p HDTV video standard. But in this case, “4K” most likely just should be “4X” (aka: Quad (4) times normal HD size).
What is this nonsense?
Is it that 4K is not defined correctly, did the technology grow independent from the naming conventions, or do the advertising companies just refuse to burden the customers with correct informations?
That is an open-ended question, but I don’t believe the issue is advertising refusing to burden consumers with information they can’t handle. Rather—if you ask me—companies are just taking advantage of the non-enforced concept of a “standard” to confuse consumers into thinking they are getting what they are not getting.
It’s just classic sales smoke and mirrors. Not as bad as CPU Mhz/Ghz speed confusion—compounded by multiple core CPUs—in the PC world, but maybe worse. If you are old enough all of these mixed and deceptive uses of terminology just can make you wistful for the days of simple, 20” CRT TVs with a modest remote control.
But wait! There’s more… Nobody can agree on 4K DRM standards!
Reading this article explains that a lot of the 4K nonsense can be traced back to DRM schemes and some companies/formats are fighting over and how streaming 4K video versus 4K physical media might contain different, competing DRM schemes which will essential creating even more 4K media variants:
In short, it’s starting to look like there could be multiple DRM systems in the market for physical media and downloadable 4K content, with different major studios making their content available on some but not others. That could be more than enough to turn consumers off to 4K content in downloadable or physical form.
Related addendum: DVD+R (plus) versus DVD-R (minus) nonsense.
And an aside, but if you remember when DVD-R’s first came out, they were just basically “DVD [dash] R’s.” But then DVD+R’s came around—as a competing recordable DVD format—the “+” resulted in some creative marketing nonsense. Such as where DVD+R media was considered superior to DVD-R because one was a “DVD [plus] R” type of media and the other (by implication) was “DVD [minus] R” and who wants to have the “minus” media when “plus” media is out there? You want “the best” right? So be positive! Get the “plus.” This kind of format branding manipulation nonsense never ends.
PS: And when the dust settled from the DVD-R/DVD+R nonsense, we ended up with the DVD±R (plus/minus) designation for DVD readers/writers so they could read and write to DVD-R media as well as DVD+R media. The fun never stops!