While most of the case studies in this volume come from biomedical research, the reader will also find studies in environmental science and marketing research. The 4th Workshop on Case Studies in Bayesian Statistics was held at Carnegie-Mellon University September 27-28, 1997. As in the past, the workshop featured both invited and contributed case studies. The former were presented in detail while the latter were presented in poster format. This volume contains the four invited case studies with the accompanying discussion as well as nine contributed papers selected by a refereeing process.