Is 4K just marketing? Here's where the name really came from

The term “4K” originated with the Digital Cinema Initiatives standard in 2005, defining a 4,096 × 2,160 pixel format for theatrical digital film; consumer “4K UHD” uses a slightly smaller 3,840 × 2,160 resolution that is exactly four times the pixel count of 1080p. The name persists because it denotes the horizontal pixel count near 4,000 and differentiates it from other formats such as DCI 4K, Ultra HD, and 2160p.