This volume constitutes the proceedings of the Fourth International Symposium on Theoretical Aspects of Computer Software (TACS 2001) held at Tohoku U- versity, Sendai, Japan in October 2001. The TACS symposium focuses on the theoretical foundations of progr- ming and their applications. As this volume shows, TACS is an international symposium, with participants from many di?erent institutions and countries. TACS 2001 was the fourth symposium in the TACS series, following TACS 91, TACS 94, and TACS 97, whose proceedings were published as Volumes 526, 789, and 1281, respectively, of...