  Abridged: The small-- and intermediate--scale structure and the fraction of
the ISM ionized by non--radiative processes is investigated in a small sample
of four local starburst galaxies, imaged with the HST WFPC2. The sample
comprises three dwarf galaxies, NGC3077, NGC4214, and NGC5253, and one giant
spiral, NGC5236 (M83). The galaxies span a range in metallicity, luminosity,
and environment, enabling the investigation of non--radiative ionization
processes in a variety of galactic conditions. For this purpose, the four
galaxies were imaged in the lines of H-beta(4861 A), [OIII](5007 A),
H-alpha(6563 A), and [SII](6717,6731 A). The non--photoionized gas in these
galaxies has been traced on scales ranging from a few tens of pc to a few
hundred pc, providing a full budget for this ionized gas component. Using the
`maximum starburst line' of Kewley et al. (2001) to discriminate between
photoionized and non--photoionized gas, we find that in all four galaxies
non--photoionization processes are responsible for a small fraction of the
total H-alpha emission, at the level of 3%--4%. The starbursts in the three
dwarf galaxies deposit a significant fraction, 70%-100%, of their mechanical
energy into the surrounding interstellar medium and require time--extended (a
few x 10^7 yr to ~10^8 yr) star formation, in order to account for the observed
luminosity of the non--photoionized gas. In the massive spiral, the
non--photoionized gas is concentrated in localized areas surrounded by active
star--formation, with no evidence for extended structures (as those in the
dwarfs). This confirms the picture that starbursts remain confined events in
massive galaxies, likely due to the deep potential well.
