The division of liquid dynamics into inertial (viscosity independent) and diffusive (viscosity dependent) components is followed to low viscosity. The previous papers in this series [J. Chem. Phys. 115, 4212 (2001); 115, 4223 (2001); 115, 4231 (2001)] found well distinguished inertial rotation, diffusive solvation and diffusive rotation of anthracene in benzyl alcohol over a range of moderate viscosities (2.7-14.4 cP). In this paper we extend those measurements to a lower viscosity range (0.55-0.82 cP) in toluene. Vibrational dynamics are almost entirely eliminated by the choice of solute and laser wavelength. The slow rotational decay component behaves normally for a diffusive process, i.e., the rotation time is linear in the viscosity. The shorter dynamics can be modeled as a poorly resolved combination of inertial rotation and diffusive solvation, but the fit solvation times are approximately a factor of two smaller than expected. This result is interpreted as a symptom of the breakdown of the inertial/diffusive distinction at intermediate time and low viscosity. The possibility that solvation and rotation become mixed under these conditions is discussed. In the <100 fs range, a very large signal is found. This peak is clearly too large and too broad to be explained by models including only two resonant electronic states. This system presents an example where these models are inadequate to deconvolve inertial solvation effects that are on a time scale similar to the pulse widths. (C) 2003 American Institute of Physics.