CALCO 2011, the 4th Conference on Algebra and Coalgebra in Computer Science, was held in Winchester, United Kingdom, August 30 -- September 2, 2011. This special issue contains extended versions of selected papers presented at the conference.

CALCO is a bi-annual event formed by joining CMCS (the International Workshop on Coalgebraic Methods in Computer Science) and WADT (the International Workshop on Algebraic Development Techniques). CALCO focuses on foundational aspects as well as both traditional and emerging uses of algebras and coalgebras in computer science, where the study of algebra and coalgebra relates to the data, process and structural aspects of software systems.

We are grateful to the University of Southampton and to the Local Organizing Committee chaired by Corina Cîrstea, for organizing the interesting and memorable event of CALCO 2011.

Proceedings of the conference with the original contributions by invited speakers and submissions selected by the Programme Committee were published by Springer-Verlag as volume 6859 of Lecture Notes in Computer Science.

The seven papers in this collection were invited by the guest editors, based on the evaluation by the Programme Committee and subreferees. All submitted papers were peer-reviewed according to the usual high standards of LMCS.

We would like to thank the authors of the papers in this special issue for their excellent submissions, and the referees for their careful and thorough work.