  We report the discovery of a 10 comoving Mpc-scale structure traced by
massive submillimeter galaxies (SMGs) at z~4.6. These galaxies are selected
from an emission line search of ALMA Band 7 observations targeting 184 luminous
submillimeter sources ($S_{850\mu{\rm m}}\geq$ 6.2 mJy) across 1.6 degrees$^2$
in the COSMOS field. We identify four [CII] emitting SMGs and two probable
[CII] emitting SMG candidates at z=4.60-4.64 with velocity-integrated
signal-to-noise ratio of SNR>8. Four of the six emitters are near-infrared
blank SMGs. After excluding one SMG whose emission line is falling at the edge
of the spectral window, all galaxies show clear velocity gradients along the
major axes that are consistent with rotating gas disks. The estimated rotation
velocities of the disks are 330-550 km s$^{-1}$ and the inferred host
dark-matter halo masses are ~2-8 $\times$ 10$^{12}$M$_{\odot}$. From their
estimated halo masses and [CII] luminosity function, we suggest that these
galaxies have a high (50-100%) duty cycle and high (~0.1) baryon conversion
efficiency (SFR relative to baryon accretion rate), and that they contribute
$\simeq$2% to the total star-formation rate density at z=4.6. These SMGs are
concentrated within just 0.3% of the full survey volume, suggesting they are
strongly clustered. The extent of this structure and the individual halo masses
suggest that these SMGs will likely evolve into members of a
~10$^{15}$M$_{\odot}$ cluster at z=0. This survey reveals synchronized dusty
starburst in massive halos at z>4, which could be driven by mergers or fed by
smooth gas accretion.
