We propose to build the fourth generation of the Distributed ASCI Supercomputer. In principle, DAS-4 will be based on a similar concept as its successful predecessors: a collection of cluster computers located at the participating institutes (Vrije Universiteit, University of Amsterdam, Delft University of Technology, Leiden University, MultimediaN, ASTRON) and integrated into a single shared testbed for experimental Computer Science research. However, the new system is also designed to address research questions that important recent developments are bringing with them, in particular: the increasing hardware diversity (multi-cores, special architectures like GPUs, MPSoCs, FPGAs); switchable photonic networks; the data explosion in e-Science and Internet applications (including sensor networks); and the increasing concern about power consumption. DAS-4 is scheduled to be operational by 2010, about four years after its predecessor (as with DAS-2 and DAS-3). The participation of ASTRON in DAS-4 is new and aims to set up and strengthen collaborations with application level computer science research fields with high computational demands. Such collaborations have proven to be highly successful in the past, with MultimediaN being a prime example. In particular, the DAS community expects to learn and benefit vastly from the computational demands in astronomy applications, to pave the way for large-scale computing solutions to be applied in other domains. Similarly, ASTRON needs a versatile experimentation platform to research new optimization strategies on a diversity of special and general purpose architectures. DAS-4 will be ideal for such investigations. This proposal describes the developments that require a new system, it summarizes earlier DAS-3 research (resulting in 36 Ph.D. theses) and then describes the research that will be done with DAS-4. Over 30 projects already exist now that will have funding by 2010 and that will use DAS-4. Next, the proposal describes the requested system, the research team, the social relevance and national interest, the implementation, and the financial aspects.