I am pleased to announce the fourth issue of 2021. As always, I would like to express my sincere appreciation for the great support that makes the continued publication of novel and high quality articles possible. Thus, I would like to thank all authors for their sound research contributions, the reviewers for their very helpful suggestions and the consortium members for their financial support.

I would also like to report on further achievements regarding our new platform. We have successfully migrated all the information of the Board of Editors and we have also started to use the new review module. Due to the cooperation with Pensoft Inc., our new platform provider, we will also be able to offer review acknowledgment on the Publons portal in the future.

In this regular issue, I am very pleased to introduce four accepted papers from three different countries and 14 involved authors.

Martin Berglund, Brink van der Merwe, and Steyn van Litsenborgh from South Africa investigate in their article regular expressions which contain lookaheads in addition to the standard operators of union, concatenation, and Kleene star. Fairouz Fakhfakh, Slim Kallel and Saoussen Cheikhrouhou from Tunisia research and discuss in their work a crucial issue in modern distributed information systems, i.e. how to verify the correctness of Cloud and Fog systems based on formal verification. Marcia Henke, Eulanda Santos, Eduardo Souto, and Altair O. Santin from Brazil introduce their enhanced spam detection system which is based on analyzing the evolution of features. And finally, also from Brazil, Marcelo Aires Vieira, Elivaldo Lozer Fracalossi Ribeiro, Daniela Barreiro Claro, and Babacar Mane investigate the challenging problem of integrating heterogeneous DaaS and DBaaS sources and explore the Data Join (DJ) method for integrating heterogeneous data.