4-screen audience measurement refers to the process of tracking and analyzing the viewership of content across four different screens: television, computer, mobile phone, and tablet. This method takes into account the changing habits of modern audiences, who consume media on multiple devices, and provides a comprehensive understanding of how and where people are engaging with content. By measuring the audience across all four screens, media companies and advertisers can gain valuable insights into the reach and impact of their content, allowing them to make more informed decisions and effectively target their audiences.