This book constitutes the refereed proceedings of the 4th International Conference on COTS-Based Software Systems, ICCBSS 2005, held in Bilbao, Spain in February 2005.The 28 revised full papers presented together with summaries of panels, workshops, tutorials, and posters were carefully reviewed and selected from numerous submissions. The papers are organized in topical sections on COTS at business, integration and interoperability, evaluation and requirements, safety and dependability, architecture and design, COTS management, and open source software.