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...