The 4?CH rovibrational manifold around 12 700 cm-1 in the electronic ground state, X~, of acetylene (C2H 2) is monitored by time-resolved infrared-ultraviolet double-resonance (IR - UV DR) spectroscopy. An IR laser pulse initially prepares rotational J states, associated with the "IR-bright" (?1 + 3?3) or (1 0 3 0 0)0 vibrational combination level, and subsequent collision-induced state-to-state energy transfer is probed by UV laser-induced fluorescence. Anharmonic, l-resonance, and Coriolis couplings affect the J states of interest, resulting in a congested rovibrational manifold that exhibits complex intramolecular dynamics. In preceding papers in this series, we have described three complementary forms of the IR - UV DR experiment (IR-scanned, UV-scanned, and kinetic) on collision-induced rovibrational satellites, comprising both regular even-?J features and unexpected odd-?J features. This paper examines an unusual collision-induced quasi-continuous background (CIQCB) effect that is apparently ubiquitous, accompanying regular even-?J rovibrational energy transfer and accounting for much of the observed collision-induced odd-?J satellite structure; certain IR-bright (1 0 3 0 0)0 rovibrational states (e.g., J = 12) are particularly prominent in this regard. We examine the mechanism of this CIQCB phenomenon in terms of a congested IR-dark rovibrational manifold that is populated by collisional transfer from the nearly isoenergetic IR-bright (1 0 3 0 0)0 submanifold. © 2006 American Chemical Society.