This book constitutes the thoroughly refereed post-proceedings of the 4th International Workshop on Product Family Engineering, PFE 2001, held in Bilbao, Spain, in October 2001. The 31 revised full papers presented together with an introduction and six session reports were carefully reviewed and selected for inclusion in the book. The papers are organized in topical sections on product issues, process issues, community issues, platform and quality solutions, diversity solutions, product validation, and process validation.