  This is the fourth installment in our series on implementing the
discontinuous Galerkin (DG) method as an open source MATLAB /GNU Octave
toolbox. Similarly to its predecessors, this part presents new features for
application developers employing DG methods and follows our strategy of relying
on fully vectorized constructs and supplying a comprehensive documentation. The
specific focus of the current work is the newly added generic problem
implementation framework and the highly customizable model-coupling interface
for multi-domain and multi-physics simulation tools based on this framework.
The functionality of the coupling interface in the FESTUNG toolbox is
illustrated using a two-way coupled free-surface / groundwater flow system as
an example application.
