This book constitutes the refereed proceedings of the 4th International Conference on E-Commerce 2003, held in Prague, Czech Republic in September 2003.The 42 revised full papers presented together with an invited paper and a position paper were carefully reviewed and selected from 108 submissions. The papers are organized in topical sections on auctions, security, agents and search, ebXML, modeling and technology, XML, design and performance, business processes, and brokering and recommender systems.