This book constitutes the refereed proceedings of the 4th International Conference on the Unified Modeling Language, 2001, held in Toronto, Canada, in October 2001.The 33 revised full papers presented together with one invited paper were carefully reviewed and selected from a total of 122 abstracts and 102 papers submitted. The papers are organized in topical sections on metamodeling, activity diagrams, OCL, architecture and patterns, analysis and testing, performance and databases, graph transformations, real-time and embedded systems, associations and ontology, statecharts, components, and use cases.