  We combine the the stellar spectral synthesis code Starburst99, the nebular
modelling code MAPPINGSIII, and a 1-D dynamical evolution model of HII regions
around massive clusters of young stars to generate improved models of the
spectral energy distribution (SED) of starburst galaxies. We introduce a
compactness parameter, C, which characterizes the specific intensity of the
radiation field at ionization fronts in HII regions, and which controls the
shape of the far-IR dust re-emission, often referred to loosely as the dust
``temperature''. We also investigate the effect of metallicity on the overall
SED and in particular, on the strength of the PAH features. We provide
templates for the mean emission produced by the young compact HII regions, the
older (10 - 100 Myr) stars and for the wavelength-dependent attenuation
produced by a foreground screen of the dust used in our model. We demonstrate
that these components may be combined to produce a excellent fit to the
observed SEDs of star formation dominated galaxies which are often used as
templates (Arp 220 and NGC 6240). This fit extends from the Lyman Limit to
wavelengths of about one mm. The methods presented in both this paper and in
the previous papers of this series allow the extraction of the physical
parameters of the starburst region (star formation rates, star formation rate
history, mean cluster mass, metallicity, dust attenuation and pressure) from
the analysis of the pan-spectral SED.
