  We have used WFPC2 on the Hubble Space Telescope to obtain F450W and F814W
images of four cD galaxies (NGC 541 in Abell 194, NGC 2832 in Abell 779, NGC
4839 in Abell 1656 and NGC 7768 in Abell 2666) in the range 5400 < cz < 8100 km
s^{-1}. For NGC 541, the HST data are supplemented by ground-based B and I
images obtained with the FORS1 on the VLT. We present surface brightness and
color profiles for each of the four galaxies, confirming their classification
as cD galaxies. Isophotal analyses reveal the presence of subarcsecond-scale
dust disks in the nuclei of NGC 541 and NGC 7768. Despite the extreme nature of
these galaxies in terms of spatial extent and luminosity, our analysis of their
globular cluster systems reveals no anomalies in terms of specific frequencies,
metallicity gradients, average metallicities, or the metallicity offset between
the globulars and the host galaxy. We show that the latter offset appears
roughly constant at \Delta [Fe/H] ~ 0.8 dex for early-type galaxies spanning a
luminosity range of roughly four orders of magnitude. We combine the globular
cluster metallicity distributions with an empirical technique described in a
series of earlier papers to investigate the form of the protogalactic mass
spectrum in these cD galaxies. We find that the observed GC metallicity
distributions are consistent with those expected if cD galaxies form through
the cannibalism of numerous galaxies and protogalactic fragments which formed
their stars and globular clusters before capture and disruption. However, the
properties of their GC systems suggest that dynamical friction is not the
primary mechanism by which these galaxies are assembled. We argue that cDs
instead form rapidly, via hierarchical merging, prior to cluster virialization.
