  We present DEIMOS spectroscopic observations of the most UV-luminous
star-forming galaxies at 3.2<z<4.6. Our sample contains galaxies with
luminosities of L*<L<7L* and is one of the largest samples to date of the most
UV-luminous galaxies at these redshifts. Our data confirm 41 star-forming
galaxies at 3.2<z<4.6 and validate the clean selection of the photometric
candidates. We find that the fraction of Lya emitting galaxies increases with
decreasing UV luminosity. We find strong evidence of large-scale outflows,
transporting the neutral/ionized gas in the interstellar medium away from the
galaxy. Galaxies exhibiting both interstellar absorption and Lya emission lines
show a significant velocity offset between the two features (200-1140 km/s). We
find tentative evidence that this measure of the outflow velocity increases
with UV luminosity and/or stellar mass. The luminosity- and mass-dependent
outflow strengths suggest that the efficiency of feedback and enrichment of the
surrounding medium depend on these parameters. We present composite spectra of
the absorption-line-only and Lya-emitting subsets of the UV luminous galaxies
at z~3.7. The composite spectra are similar to those of lower-z and
lower-luminosity LBGs samples, but with some subtle differences. Analyses of
the composite spectra suggest that the UV luminous LBGs at z~3.7 may have a
higher covering fraction of absorbing gas, and may be older than their lower-z
and lower-luminosity counterparts. In addition, we have discovered 5 galaxies
that belong to a massive overdensity at z=3.78. Finally, two galaxies each show
two distinct sets of interstellar absorption features. The latter may be a sign
of a final stage of major merger, or clumpy disk formation. Their presence
implies that frequency of such sources among our luminous z~3.7 LBGs may be an
order of magnitude higher than in lower redshift and lower luminosity samples.
