This book constitutes the refereed proceedings of the Fourth International Symposium on Search-Based Software Engineering, SSBSE 2012, held in Riva del Garda, Italy in collocation with the 28th IEEE International Conference on Software Maintenance. The 15 revised full papers, 3 revised short papers, and 2 papers of the graduate track presented together with 2 keynote talks and 1 tutorial paper were carefully reviewed and selected from 38 initial submissions. Search-based Software Engineering (SBSE) studies the application of meta-heuristic optimization techniques to various software engineering problems, ranging from requirements engineering to software testing and maintenance. The papers present current research in all areas of Search Based Software Engineering, including theoretical work, research on SBSE applications, empirical studies, and reports on industrial experience.