This book constitutes the refereed proceedings of the 4th International Symposium on Foundations of Information and Knowledge Systems, held in February 2006. The 14 revised full papers presented together with three revised short papers and one invited paper were carefully reviewed and selected from 54 submissions. Among the topics covered are the theoretical foundations of information and knowledge systems, as well as mathematical fields such as discrete mathematics, combinatorics, logics and finite model theory, and applications thereof for research on database and knowledge base theory.