This book constitutes the refereed proceedings of the 4th International Conference on Foundations of Software Science and Computation Structures, FOSSACS 2001, held in Genova, Italy in April 2001. The 25 revised full papers presented together with an invited paper and a tool presentation paper were carefully reviewed and selected from a total of 63 submissions. Among the topics covered are algebraic, categorical, logical, and geometric theories, models, and methods supporting the specification, synthesis, verification, analysis, and transformation of sequential, concurrent, distributed and mobile programs and software systems.