I will present a detailed census of galaxies in and around a spectroscopically confirmed Coma analog observed at z=3.8. Diverse types of constituents identified in the field include low-mass line emitters, massive star-forming galaxies, and ultra-massive galaxies which may have already halted their star formation. The sky distribution of the star-forming galaxies suggests a significant overdensity (space density contrast of ~8+/-2), which is offset from the previously confirmed members by 3-4 Mpc. Candidate quiescent and post-starburst galaxies are also found in large excess. The curious configuration of their distribution hints that the true extent of this structure may be larger than previously known, making this a singularly large cosmic structure rarely seen even in large cosmological simulations. Among the members is one of the largest star-forming galaxies discovered to date, which may be a nearly equal-mass merger involving two super-L* galaxies in its early stage. I will discuss the main mode of galaxy formation in dense protocluster environments.
