This collection of papers contributes results to all aspects in knowledge discovery and data mining on the research level as well as on machine learning, information systems, the Internet, statistics, knowledge acquisition, data visualization, software re-engineering, and knowledge-based systems.

The 4th International Conference on Hybrid Artificial Intelligence Systems (HAIS 2009), as the name suggests, attracted researchers who are involved in developing and applying symbolic and sub-symbolic techniques aimed at the construction of highly robust and reliable problem-solving techniques, and bringing the most relevant achievements in this field. Hybrid intelligent systems have become increasingly po- lar given their capabilities to handle a broad spectrum of real-world complex problems which come with inherent imprecision, uncertainty and vagueness, hi- dimensionality, and...