  Recent progress in the field of topological states of matter(1,2) has largely
been initiated by the discovery of bismuth and antimony chalcogenide bulk
topological insulators (TIs)(3-6), followed by closely related ternary
compounds(7-16) and predictions of several weak TIs(17-19). However, both the
conceptual richness of Z$_2$ classification of TIs as well as their structural
and compositional diversity are far from being fully exploited. Here, a new
Z$_2$ topological insulator is theoretically predicted and experimentally
confirmed in the $\beta$-phase of quasi-one-dimensional bismuth iodide
Bi$_4$I$_4$. The electronic structure of $\beta$-Bi$_4$I$_4$, characterized by
Z$_2$ invariants (1;110), is in proximity of both the weak TI phase (0;001) and
the trivial insulator phase (0;000). Our angle-resolved photoemission
spectroscopy measurements on the (001) surface reveal a highly anisotropic
band-crossing feature located at the point of the surface Brillouin zone and
showing no dispersion with the photon energy, thus being fully consistent with
the theoretical prediction.
