What bothers me about 4k is the switching of terms. 4k makes it sound like it has 4 times the vertical resolution of 1080p, but for pure marketing reasons, suddenly instead of the 4k referring to vertical resolution as 1080 did, it refers to horizontal resolution, so is really only twice the horizontal resolution of the 1920 that 1080p had. Overall that's 4 times the pixels because of squaring, but still!
We really ought to insist on using more correct terms like UHD (Ultra High Definition = 3840×2160 pixels) and only referring to "true" 4K cinema (4096 × 2160 pixels) content/equipment as 4K.
a) Number of pixels in scientific notation
b) Alternatively log base 10 of number of pixels