This book constítutes the thoroughly refereed postproceedings of the 4th International Conference on the Advanced Encryption Standard, AES 2004, held in Bonn, Germany in May 2004. The 10 revised full papers presented together with an introductory survey and 4 invited papers by leading researchers were carefully selected during two rounds of reviewing and improvement. The papers are organized in topical sections on cryptanalytic attacks and related topics, algebraic attacks and related results, hardware implementations, and other topics. All in all, the papers constitute a most up-to-date assessment of the state of the art of data encryption using the Advanced Encryption Standard AES, the de facto world standard for data encryption.