The Fourth International Workshop on Maintenance and Evolution of Service-Oriented Systems (MESOA 2010), organized by members of the Carnegie Mellon Software Engineering Institute's technical staff, was held at the 26th International Conference on Software Maintenance (ICSM 2010) in Timisoara, Romania, on September 17, 2010. The goal for MESOA 2010 was to share current research efforts and discuss emerging technologies in the maintenance and evolution of service-oriented systems. A second goal of the workshop was to identify areas of future work needed to address existing gaps and problems in the taxonomy of research topics in service-oriented architecture (SOA). This report summarizes the workshop and includes the accepted papers that were the basis for the presentations given during the workshop. Topics include using simulation models to evolve business processes, a requirements-driven framework for root cause analysis in SOA environments, SOA integration as an alternative to source migration, proactive adaptation as illustrated by the S-Cube service life cycle, a dynamic framework for quality web-service discovery, a characterization of policies that govern SOAs, and context-driven adaptive monitoring for supporting SOA governance. The report concludes with highlights from the discussions among workshop attendees.