This book constitutes the refereed proceedings of the 4th International Conference on Discovery Science, DS 2001, held in Washington, DC, USA, in November 2001. The 30 revised full papers presented together with 3 invited papers and 11 posters were carefully reviewed and selected from numerous submissions. Among the topics addressed in their relation to discovery science are inference, algorithmic learning, heuristic search, database management, data mining, inductive logic programming, information agents, information retrieval, information visualization, etc.