  We present a detailed census of galaxies in and around PC217.96+32.3, a
spectroscopically confirmed Coma analog at z=3.78. Diverse galaxy types
identified in the field include Lya emitters (LAEs), massive star-forming
galaxies, and ultra-massive galaxies (log (Mstar/Msun)>= 11) which may have
already halted their star formation. The sky distribution of the star-forming
galaxies suggests the presence of a significant overdensity (delta_g=8+/-2),
which is spatially offset from the previously confirmed members by 3-4 Mpc to
the west. Candidate quiescent and post-starburst galaxies are also found in
large excess (a factor of ~ 8-15 higher surface density than the field)
although their redshifts are less certain. We estimate that the total enclosed
mass traced by star-forming galaxy candidates is roughly comparable to that of
PC217.96+32.3 traced by the LAEs. We speculate that the true extent of
PC217.96+32.3 may be larger than previously known, a half of which is missed by
our LAE selection. Alternatively, the newly discovered overdensity may belong
to another Coma progenitor not associated with PC217.96+32.3. Expectations from
theory suggest that both scenarios are equally unlikely (<1%), particularly in
the cosmic volume probed in our survey. If confirmed as a single structure, its
total mass will be well in excess of Coma, making this a singularly large
cosmic structure rarely seen even in large cosmological simulations. Finally,
we find that the protocluster galaxies follow the same SFR-M_star scaling
relation as the field galaxies, suggesting that the environmental effect at z~4
is a subtle one at best for normal star-forming galaxies.
