This book constitutes the thoroughly refereed post-conference proceedings of the Fourth International Conference on Fundamentals of Software Engineering, FSEN 2011, held in Tehran, Iran, in April 2011. The 19 revised full papers and 5 revised short papers presented together with 3 poster presentations were carefully reviewed and selected from 64 submissions. The papers are organized in topical section on models of programs and systems, software specification, validation and verification, software architectures and their description languages, object and multi-agent systems, CASE tools and tool integration, model checking and theorem proving, and Integration of different formal methods.