This volume constitutes the strictly refereed post-workshop proceedings of the Fourth International Workshop on Fast Software Encryption, FSE'97, held in Haifa, Israel, in January 1997. The 23 full papers presented were carefully selected from 44 submissions and revised for inclusion in the book. Also contained is a summary of a panel discussion. The papers are organized in sections on cryptanalysis, blockciphers, stream ciphers, message authentication codes, modes of operation, and fast software encryption. Particular emphasis is placed on applicability and implementation issues of fast cryptography.