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 Ly \alpha emitters ( LAEs ) , massive star-forming galaxies , and ultra-massive galaxies ( \gtrsim 10 ^ { 11 } M _ { \odot } ) which may have already halted their star formation .
The sky distribution of the star-forming galaxies suggests the presence of a significant overdensity ( \delta _ { SFG } \approx 8 \pm 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 \sim 8–15 higher surface density than the field ) although their redshifts are less certain .
We estimate that the total enclosed mass traced by candidate star-forming galaxies is roughly comparable to that of PC217.96+32.3 traced by the LAEs .
We speculate that the true extent of P217.96+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 % ) , 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 an exceptionally large cosmic structure rarely seen even in large cosmological simulations .
Finally , we find that the protocluster galaxies follow the same SFR- M _ { * } scaling relation as the field galaxies , suggesting that the environmental effect at z \sim 4 is a subtle one at best for normal star-forming galaxies .