This book constitutes the thoroughly refereed post-proceedings of the 4th International Andrei Ershov Memorial Conference, PSI 2001, held in Akademgorodok, Novosibirsk, Russia, in July 2001. The 50 revised papers presented together with 2 invited memorial papers devoted to the work of Andrei Ershov were carefully selected during 2 rounds of reviewing and improvement. The book offers topical sections on computing and algorithms, logical methods, verification, program transformation and synthesis, semantics and types, processes and concurrency, UML specification, Petri nets, testing, software construction, data and knowledge bases, logic programming, constraint programming, program analysis, and language implementation.