  We use the database from Paper III to quantify the global and nuclear
properties of emission-line nuclei in the Palomar spectroscopic survey of
nearby galaxies. We show that the host galaxies of Seyferts, LINERs, and
transition objects share remarkably similar large-scale properties and local
environments. The distinguishing traits emerge on nuclear scales. Compared with
LINERs, Seyfert nuclei are an order of magnitude more luminous and exhibit
higher electron densities and internal extinction. We suggest that Seyfert
galaxies possess characteristically more gas-rich circumnuclear regions, and
hence a more abundant fuel reservoir and plausibly higher accretion rates. The
differences between the ionization state of the narrow emission-line regions of
Seyferts and LINERs can be partly explained by the differences in their nebular
properties. Transition-type objects are consistent with being composite
(LINER/\hii) systems. With very few exceptions, the stellar population within
the central few hundred parsecs of the host galaxies is uniformly old, a
finding that presents a serious challenge to starburst or post-starburst models
for these objects. Seyferts and LINERs have virtually indistinguishable
velocity fields as inferred from their line widths and line asymmetries. All
three classes of objects obey a strong correlation between line width and line
luminosity. We argue that the angular momentum content of circumnuclear gas may
be an important factor in determining whether a nucleus becomes active.
Finally, we discuss some possible complications for the unification model of
Seyfert galaxies posed by our observations. (Abridged)
