This volume contains the proceedings of DLES4, the fourth international ERCOFTAC workshop on Direct and Large-Eddy Simulation which was held at the University of Twente, July 18-20, 2001. It contains an exposition and discussion of modeling and simulation approaches for complex flows. Fundamental turbulence - and modeling issues and elements from modern numerical analysis are at the heart of this field of interest. Present-day DNS and LES tend to be extended toward flows of realistic complexity, at practically relevant flow conditions and with the inclusion of additional complex physical phenomena such as arise for example in combustion research, environmental flows, multiphase applications and process-engineering. These extensions of DNS and LES are illustrated in the contributions, while an ongoing interest remains with sophisticated modeling and numerical approaches, paying attention to physical, mathematical and engineering aspects.