We welcome you to the fourth ACM Multimedia Systems conference (MMSys). This conference focuses on multimedia computing and networking, lying at the intersection of digital media and the systems (applications, architectures, networking, operating systems) that handle them. The goal is to cut across a diverse set of computer science subfields in order to achieve a longitudinal perspective on the challenges that multimedia data types present. This provides a unique opportunity to view the intersections and interplay of the various approaches and solutions developed across these domains to deal with multimedia data types. Furthermore, MMSys provides an avenue for communicating research that addresses multimedia systems holistically.