The fourth Machine Learning Summer School was held in Berder Island, France between the 12th and the 25th of September, 2004. More than 100 students and researchers from 20 countries interested in Machine Learning attended. This years' summer school presented some of the topics which are at the core of modern Learning Theory. 15 distinguished authorities from the field gave 14 courses in slots from 4 to 8 hours. In addition, many evening talks which were focused on additional topics were presented. There were also three practical sessions organized providing a 'hands-on' experience of working with Machine Learning algorithms.