TheSDLandMSC(SAM)workshop, heldeverytwoyears, providesanopend- cussionarenaontopicsrelatedtothemodellingandanalysisofreactivesystems, distributed systems, and real-time systems (e.g., telecommunications, auto- tive, aerospace, and Web-based applications). The SAM workshop is a place for intensive discussions enabling the unfolding of ideas for the future devel- ment and application of SDL and MSC, and of related languages: ASN.1, eODL, TTCN, UML, and URN. The fourth instance of this workshop was held at the University of Ottawa, Canada, from June 1 to June 4, 2004 (http: //www.site.uottawa.ca/sam04/). Itwasco-organizedbytheUniversityofOttawa, theSDLForumSociety, andthe International Telecommunication Union (ITU-T). SAM 2004 was also sponsored by SOLINET, SAFIRE-SDL, and Telelogic AB. The workshop welcomed 60 participants from 10 di?erent countries, including SDL Forum members, tool vendors, standardizers, industrial users, and researchers. In 2004, the program was composed of 21 papers, two panel sessions, one tutorial, several posters, and the third edition of the SDL design contest. The papers were selected by the Program Committee from 46 submissions. After postworkshoprevisions, asecondroundofreviewledtotheselectionof19papers for publication in this volume of Lecture Notes in Computer Science. Since the theme proposed for 2004 was Security Analysis and Modelling, the workshop started with a full-day tutorial on black-box security protocols, given by Sjouke Mauw and Cas Cremers. This tutorial introduced the basics of security protocols (which are "three-line programs that people still manage to get wrong") and ways of preventing many types of attacks based on a security model, veri?cation, and formal modelling and analysis. Many of the models were expressed as message sequence charts annotated with security properties.