While DCI 4K is a standard with 4,096 pixels of width, you’re correct that the HD standard (and therefore what is relevant to the discussion here) has always be

While DCI 4K is a standard with 4,096 pixels of width, you’re correct that the HD standard (and therefore what is relevant to the discussion here) has always been UHD 4K and 3840 pixels wide. DCI is relevant for movie industry professionals only, as these are the dimensions used for projection devices and (potentially) their content.