  We report the discovery and spectroscopic confirmation of a very large
star-forming Lyman Break galaxy, G6025, at z_spec=3.721+/-0.003. In the
rest-frame ~2100A, G6025 subtends ~24 kpc in physical extent when measured from
the 1.5-sigma isophote, in agreement with the parametric size measurements
which yield the half-light radius of 4.9+/-0.5 kpc and the semi-major axis of
12.5+/-0.1 kpc. G6025 is also very UV-luminous (~5L*(z~4}) and young (~140+/-60
Myr). Despite its unusual size and luminosity, the stellar population
parameters and dust reddening (M_star~M*(z~4)$, and E(B-V)=0.18+/-0.05)
estimated from the integrated light, are similar to those of smaller galaxies
at comparable redshifts. The ground-based morphology and spectroscopy show two
dominant components, both located off-center, embedded in more diffuse
emission. We speculate that G6025 may be a scaled-up version of chain galaxies
seen in deep HST imaging, or alternatively, a nearly equal-mass merger
involving two super-L* galaxies in its early stage. G6025 lies close to but not
within a known massive protocluster at z=3.78. We find four companions within 6
Mpc from G6025, two of which lie within 1.6 Mpc. While the limited sensitivity
of the existing spectroscopy does not allow us to robustly characterize the
local environment of G6025, it likely resides in a locally overdense
environment. The luminosity, size, and youth of G6025 make it uniquely suited
to study the early formation of massive galaxies in the universe.

Astrophysics of Galaxies