This book constitutes the refereed proceedings of the 4th International Conference on Sequences and Their Applications, SETA 2006. The book presents 32 revised full papers together with 4 invited lectures. The papers are organized in topical sections on linear complexity of sequences, correlation of sequences, stream ciphers and transforms, topics in complexities of sequences, multi-sequence synthesis, sequences and combinatorics, FCSR sequences, aperiodic correlation and applications, and boolean functions, and more.