The 4th International Workshop on Modeling the Ocean (IWMO) was held on May 21 24, 2012 in the vibrant city of Yokohama on the Tokyo Bay, Japan. Over 100 researchers worldwide participated in the Workshop, which included two keynote lecturers, by Professor Mellor of Princeton University and Professor Yamagata of the University of Tokyo, and more than 80 oral presentations and 20 posters. The collection of 11 papers in this Topical Collection was selected from a total of 22 originally submitted after they underwent the usual process of reviews and revision. Ezer and Oey use the results of high-resolution numerical experiments based on the Princeton Ocean Model (POM) of the Bering Sea to study the dynamics of strait flows. Miyama and Miyazawa address why and how the Kuroshio is observed to accelerate as it flows past Cape Shionomisaki at the most southern tip of the island of Honshu, Japan. Gao modeled the circulation in the Gulf of Tonkin using the Princeton Ocean Model forced by surface and lateral boundary conditions and tidal forcing.