  [Abridged] We investigate the average physical properties and star formation
histories of the most UV-luminous star-forming galaxies at z~3.7. Our results
are derived from analyses of the average spectral energy distributions (SEDs),
constructed from stacked optical to infrared photometry, of a sample of the
1,902 most UV-luminous star-forming galaxies found in 5.3 square degrees of the
NOAO Deep Wide-Field Survey. We bin the sample according to UV luminosity, and
find that the shape of the average SED in the rest-frame optical and infrared
is fairly constant with UV luminosity: i.e., more UV luminous galaxies are, on
average, also more luminous at longer wavelengths. In the rest-UV, however, the
spectral slope (measured at 0.13-0.28 um) rises steeply with the median UV
luminosity from -1.8 at L L* to -1.2 in the brightest bin (L~4-5L*). We use
population synthesis analyses to derive the average physical properties of
these galaxies and find that: (1) L_UV, and thus star formation rates (SFRs),
scale closely with stellar mass such that more UV-luminous galaxies are also
more massive; (2) The median ages indicate that the stellar populations are
relatively young (200-400 Myr) and show little correlation with UV luminosity;
and (3) More UV-luminous galaxies are dustier than their less-luminous
counterparts, such that L~4-5L* galaxies are extincted up to A(1600)=2 mag
while L L* galaxies have A(1600)=0.7-1.5 mag. Based on these observations, we
argue that the average star formation histories of UV-luminous galaxies are
better described by models in which SFR increases with time in order to
simultaneously reproduce the tight correlation between the observed SFR and
stellar mass, and the universally young ages of these galaxies. We demonstrate
the potential of measurements of the SFR-M* relation at multiple redshifts to
discriminate between simple models of star formation histories.
