TAPSOFT '91 is the Fourth International Joint Conference on Theory and Practice of Software Development. It was held in Brighton, April 8-12, 1991, and was organized by the Department of Computing, Imperial College, London. The proceedings of TAPSOFT '91 are organized into three parts: - Colloquium on Trees in Algebra and Programming (CAAP); - Advances in Distributed Computing (ADC); - Colloquium on Combining Paradigms for Software Development (CCPSD). The first part (CAAP) is contained in Vol. 1, the other two parts constitute Vol. 2. CAAP '91 focuses on the following...