  Conventional topological insulators and superconductors have topologically
protected nodal points on their boundaries, and the recent interests in
nodal-line semimetals only concerned bulk band structures. Here, we present a
novel four-dimensional topological insulator protected by an anti-unitary
reflection symmetry, whose boundary band has a single $PT$-symmetric nodal line
with double topological charges. Inspired by the recent experimental
realization of the four-dimensional quantum Hall effect, we also propose a
cold-atom system which realizes the novel topological insulator with tunable
parameters as extra dimensions.
