This book constitutes the refereed proceedings of the 4th International Symposium on Unifying Theories of Programming, UTP 2012, held in Paris, France, in August 2012, co-located with the 18th International Symposium on Formal Methods, FM 2012. The 8 revised full papers presented together with 2 invited talks and one invited lecture were carefully reviewed and selected from 13 submissions.

Unifying Theories of Undefinedness in UTP.- Unifying Theories of Programming with Monads.- Circus Time with Reactive Designs.- Algebra Unifies Operational Calculi.- A Probabilistic Theory of Designs Based on Distributions.- The Logic of U-(TP)2.- Conscriptions: A New Relational Model for Sequential Computations.- Mechanical Approach to Linking Operational Semantics and Algebraic Semantics for Verilog Using Maude.- Unifying Operational Semantics with Algebraic Semantics for Instantaneous Reactions.- Higher-Order UTP for a Theory of Methods.- Denotational Semantics for a Probabilistic Timed Shared-Variable Language.