The Fourth International Conference on Reliable Software Technologies, Ada- Europe'99, took place in Santander, Spain, from June 7 to 11, 1999. It was sponsored by Ada Europe, the European federation of national Ada societies, in cooperation with ACM SIGAda and Ada Spain, and it was organized by members of the University of Cantabria and the Technical University of Madrid, in Spain. This was the 19th consecutive year of Ada Europe conferences, which have always been the main Ada events in Europe, with their counterparts being the ACM SIGAda conferences in the USA (formerly Tri Ada). The conference is not just devoted to the Ada language, but rather to the more general area of reliable software technologies. In this sense, there are papers on formal methods, testing, software architectures and design, software engineering tools, etc. We believe that the role of reliable software technologies is becoming increasingly important, as computer applications control more and more of our everyday systems. The goal of our conference is to contribute to advancing the state of the art of all the technologies that help us in achieving better and more reliable software at a lower overall cost.