It is quite widely accepted that 3840x2160 is "4K", eg. quoting wikipedia:
> The SMPTE first released Standard 2036 for UHDTV in 2007. UHDTV was defined as having two levels called UHDTV1 (3840×2160 or 4K UHDTV) and UHDTV2 (7680x4320 or 8K UHDTV). http://en.wikipedia.org/wiki/4K_UHDTV
> The SMPTE first released Standard 2036 for UHDTV in 2007. UHDTV was defined as having two levels called UHDTV1 (3840×2160 or 4K UHDTV) and UHDTV2 (7680x4320 or 8K UHDTV). http://en.wikipedia.org/wiki/4K_UHDTV
and
> The television industry has adopted ultra high definition television (UHDTV) as its 4K standard. http://en.wikipedia.org/wiki/4K_resolution