This book contains the proceedings of the Fourth International Workshop on Product Family Engineering, PFE-4, held in Bilbao, Spain, October 3 5, 2001. This workshop was the fourth in a series started in 1996, with the same s- ject, software product-family engineering. Proceedings of the second and third workshops have been published as LNCS 1429 and LNCS 1951. The workshops were organized within co-operation projects of European - dustry, the ?rst two by ARES (Esprit IV 20.477) 1995 1999. This project had three industrial and three academic partners, and focused on software archit-...