  Using data from the Southern Galactic Plane Survey we have measured the
spatial power spectrum of the interstellar neutral atomic hydrogen in the
fourth Galactic quadrant. This function shows the same power law behavior that
has been found for H I in the second quadrant of the Milky Way and in the
Magellanic Clouds, with the same slope. When we average over velocity intervals
broader than the typical small-scale velocity dispersion, we find that the
slope steepens, from approx. -3 to -4 for the warm gas, as predicted by
theories of interstellar turbulence if the column density fluctuations are
dominated by variations in the gas density on small spatial scales. The cool
gas shows a different increase of slope, that suggests that it is in the regime
of turbulence dominated by fluctuations in the velocity field. Overall, these
results confirm that the small scale structure and motions in the neutral
atomic medium are well described by a turbulent cascade of kinetic energy.
