  We study lattice Hamiltonian realisations of (3+1)d Dijkgraaf-Witten theory
with gapped boundaries. In addition to the bulk loop-like excitations, the
Hamiltonian yields bulk dyonic string-like excitations that terminate at gapped
boundaries. Using a tube algebra approach, we classify such excitations and
derive the corresponding representation theory. Via a dimensional reduction
argument, we relate this tube algebra to that describing (2+1)d boundary
point-like excitations at interfaces between two gapped boundaries. Such
point-like excitations are well known to be encoded into a bicategory of module
categories over the input fusion category. Exploiting this correspondence, we
define a bicategory that encodes the string-like excitations ending at gapped
boundaries, showing that it is a sub-bicategory of the centre of the input
bicategory of group-graded 2-vector spaces. In the process, we explain how
gapped boundaries in (3+1)d can be labelled by so-called pseudo-algebra objects
over this input bicategory.
