We report broad-band imaging observations which reveal a circum-nuclear gas ring in NGC 4151. The ring is delineated by two arc-like features probably resulting from dust extinction of the background stellar continuum. These features are distributed along opposite quadrants of an elliptical ring which has a semi-major axis of length 1.8kpc and is approximately aligned with the NE-SW axis of elongation of the extended emission line region. We discuss this ring in relation to the spatial distribution and kinematics of both ionized and neutral gas components as revealed by published optical spectroscopy and 21-cm observations. These data allow us to form a coherent picture of the gas flows in the circum-nuclear environment of NGC 4151 which is strikingly similar to numerical simulations of streaming induced by barred potentials in which the x2 family of orbits is well developed. On this basis, we propose that the distribution of gaseous material in the galactic bar of NGC 4151 traces the path by which the gas is transported towards the active nucleus. The alignment between the circum-nuclear ring and the ENLR can then be seen as a strong indication that the collimation of the ionizing radiation field of the active nucleus is related to the dynamical parameters of the galactic bar. Specifically, we propose that bar-driven gas flows create the opaque torus which is believed to collimate the radiation field.