For a long time we specified displays by their vertical dimension -- 480p, 720p, 1080p.
Then the marketing guys came along and decided that the horizontal dimension sounds bigger. If we stuck with the less-bullshitty way of doing things and kept comparisons 1:1, we'd call 3840x2160 displays 2160p or "2K" displays, but instead, the marketing people decided that we're going to change things to horizontal and called 3840x2160 "4K".
For a long time we specified displays by their vertical dimension -- 480p, 720p, 1080p.
Then the marketing guys came along and decided that the horizontal dimension sounds bigger. If we stuck with the less-bullshitty way of doing things and kept comparisons 1:1, we'd call 3840x2160 displays 2160p or "2K" displays, but instead, the marketing people decided that we're going to change things to horizontal and called 3840x2160 "4K".