This book constitutes the refereed proceedings of the 4th International Conference on Frontiers of Combining Systems, FroCoS 2002, held in Santa Margherita Ligure, Italy, in April 2002. The 14 revised full papers presented together with 3 invited papers were carefully reviewed and selected from 35 submissions. Among the topics covered are combination of logics, combination of constraint solving techniques, combination of decision procedures, combination problems in verification, modular problems of theorem proving, and the integration of decision procedures and other solving processes into constraint programming and deduction systems.