This book constitutes the refereed proceedings of the 4th International Conference on Sequences and Their Applications, SETA 2006, held in Beijing in September 2006.The 32 revised full papers presented together with 4 invited lectures were carefully reviewed and selected from 70 submissions. The papers are organized in topical sections on linear complexity of sequences, correlation of sequences, stream ciphers and transforms, topics in complexities of sequences, linear/nonlinear feedback shift register sequences, multi-sequence synthesis, filtering sequences and pseudorandom sequence generators, sequences and combinatorics, FCSR sequences, aperiodic correlation and applications, and boolean functions.