This volume contains the papers presented at the 4th Refinement Workshop, organized by the Formal Aspects group of the British Computer Society, and held in Cambridge on January 9-11, 1991. "Refinement" is the term used to describe the systematic and formal methodology of specifying hardware and software and transforming the specifications into designs and implementations. The value of formal methods in producing reliable hardware and software is now widely appreciated among both academics and many workers in industry, although such methods - and in particular their application to industrial-scale problems - are still in their infancy. This workshop brought together European workers in the field to present tutorials on the state of the art in both theory and industrial applications. The papers cover the development of both sequential and concurrent systems and the theoretical ideas and support tools for their effective use. This volume will be of interest to researchers in formal methods and software engineers wanting to keep track of developments in the application of formal methods in industry.