This book constitutes the thoroughly refereed post-proceedings of the 4th International Information Hiding Workshop, IHW 2001, held in Pittsburgh, PA, USA, in April 2001. The 29 revised full papers presented were carefully selected during two rounds of reviewing and revision. All current issues in information hiding are addressed including watermarking and fingerprinting of digitial audio, still image and video; anonymous communications; steganography and subliminal channels; covert channels; and database inference channels.