This volume contains papers presented at the Fourth European Conference on ComputationalLearningTheory,whichwasheldatNordkirchenCastle,inNo- kirchen, NRW, Germany, from March 29 to 31, 1999. This conference is the fourth in a series of bi-annual conferences established in 1993. TheEuroCOLTconferencesarefocusedontheanalysisoflearningalgorithms and the theory of machine learning, and bring together researchers from a wide variety of related elds. Some of the issues and topics that are addressed include the sample and computational complexity of learning speci c model classes, frameworks modeling the interaction between the learner, teacher and the en- ronment (such as learning with queries, learning control policies and inductive inference),learningwithcomplexmodels(suchasdecisiontrees,neuralnetworks, and support vector machines), learning with minimal prior assumptions (such as mistake-bound models, universal prediction, and agnostic learning), and the study of model selection techniques. We hope that these conferences stimulate an interdisciplinary scienti c interaction that will be fruitful in all represented elds. Thirty- ve papers were submitted to the program committee for conside- tion, and twenty-one of these were accepted for presentation at the conference and publication in these proceedings. In addition, Robert Schapire (AT & T Labs), and Richard Sutton (AT & T Labs) were invited to give lectures and contribute a written version to these proceedings. There were a number of other joint events including a banquet and an excursion to Munster . The IFIP WG 1.4 Scholarship was awarded to Andra s Antos for his paper Lower bounds on the rate of convergence of nonparametric pattern recognition".