According to Wikipedia:
The name “4K resolution” refers to a horizontal resolution of approximately 4,000 pixels.
However, I believed that 4K is called like this simply because it has four times the resolution of a 1080p video, like this:
and, well, 1080 is practically 1024, which is a round number 1K.
Personally, the Wikipedia's explanation doesn't make much sense to me: the resolution of a piece of video is usually specified by two things: height and ratio; it is very unusual to see any mention of the width.
So what is the real origin of “4K” term?