This volume contains papers presented at the fourth working conference on Communications and Multimedia Security (CMS'99), held in Leuven, Belgium from September 20-21, 1999. The Conference, arrangedjointly by Technical Committees 11 and 6 of the International Federation of Information Processing (IFIP), was organized by the Department of Electrical Engineering of the Katholieke Universiteit Leuven. The name Communications and Multimedia Security was used for the first time in 1995, when Reinhard Posch organized the first in this series of conferences in Graz, Austria, following up on the previously national (Austrian) IT Sicherheit conferences held in Klagenfurt (1993) and Vienna (1994). In 1996, CMS took place in Essen, Germany; in 1997 the conference moved to Athens, Greece. The Conference aims to provide an international forum for presentations and discussions on protocols and techniques for providing secure information networks. The contributions in this volume review the state-of the-art in communications and multimedia security, and discuss practical of topics experiences and new developments. They cover a wide spectrum inc1uding network security, web security, protocols for entity authentication and key agreement, protocols for mobile environments, applied cryptology, watermarking, smart cards, and legal aspects of digital signatures.