What does the 4K mean? Is its pixel more than ultra HD? If 4K is four times larger than 1080P, does it mean 4K equals to 4320P? The above questions are answered in order: it depends, sometimes and no.
720P, 1080P and so on mean the total rows of video pixel, for example, 720P means there are 720 pixels in the video while 1080P means there are 1080 pixels in total in the video, pixel of the camera with 1080P resolution ratio is always 1920*1080. P itself means progressive scanning, it is the abbreviation of progressive, which is relative to interlaced.
2k, 4k and so on mean the total columns of video pixel, such as 4K means there are 4000 columns in the video, specially is 3840 or 4096 columns. Pixel of camera with 4K resolution ratio is 3840*2160 or 4096*2160.
MP is sum of pixel, it refers to the result after P and K multiplies. For example, 1080P camera is multiplied by 1920 pixel and 1080 pixel, as a result, it gets 2MP, which is the sum of pixel.
Resolution ratio used to be described by vertical resolution, so 1080P means vertical resolution. Nearly all of aspect ratio of HD display is 1.78：1 ( 16×9 or wide screen), that means its vertical resolution is 1920 pixels ( 1920×1080).
This is why people are confused about resolution ratio now, because in the last decades, what we talked about resolution ratio is about vertical resolution, now it becomes 4K, which is horizontal resolution.
Therefore, 1080P does not equal to 1K, it just like ultra high definition equals to 4K, 1080P equals to 2K. That is to say, 1920×1080 is close to 2K that is described by DCI. Even so, most of people don't call 1080P as 2K, they call it as 1080P or full high definition. The number of 720P pixel is half of 1080P.
Of course, before 4K becomes popular, we have never seen the word 2K, it refers to standard format. Most cinemas uses 2K laser projectors
( some projectors may have lower resolution ratio. As well, according to the exposition of DCI, it just refer to its horizontal resolution is 2018, vertical resolution is not assigned.