"Unifying Themes in Complex Systems IV" is a comprehensive collection of 43 selected papers presented at the fourth International Conference on Complex Systems, which took place in June 2002 in Boston. This conference brought together over 500 professionals from various disciplines, including biology, physics, engineering, and social sciences, to promote interdisciplinary exchange and develop a deeper understanding of the common properties of complex systems. The contributions included in this volume cover a wide range of topics, including cellular automata, neurology, evolution, network dynamics, and urban planning. The publication reflects the efforts of the New England Complex Systems Institute, which has been dedicated to advancing the science of complex systems and their applications for over a decade. The editors, Ali A. Minai and Yaneer Bar-Yam, bring their extensive expertise in the field to highlight the relevance and practical applications of complex systems.