This book constitutes the thoroughly refereed post-proceedings of the 4th International Workshop on Software Composition, SC 2005, held in Edinburgh, UK, in April 2005. The workshop was co-located with the ETAPS 2005 conference. The 9 revised full papers and 4 revised short papers presented together with a keynote paper were carefully reviewed and selected from 41 submissions. The papers are organized in topical sections on dynamic languages, component composition, component contracts and protocols, and component adaptation and configuration.