  Levin-Wen models are microscopic spin models for topological phases of matter
in (2+1)-dimension. We introduce a generalization of such models to
(3+1)-dimension based on unitary braided fusion categories, also known as
unitary premodular categories. We discuss the ground state degeneracy on
3-manifolds and statistics of excitations which include both points and defect
loops. Potential connections with recently proposed fractional topological
insulators and projective ribbon permutation statistics are described.
