Higher power and energy dissipation in recent years has led to an increase in packaging cost and a decrease in product reliability in all segments of computing market, and a significant reduction of battery life in portable systems. The fourth workshop on Power-Aware Computer Systems, as before, will bring together experts from key areas in computer system design, interested in power-aware and low-power design and evlauation. PACS'04 is organized by Babak Falsafi and T. N. Vijaykumar. Please send questions to pacs04@ece.cmu.edu.