Protoclusters are good laboratories for galaxy evolution. The HSC-SSP has uniquely enabled systematic searches of high-z protoclusters. Based on its S21A data release, a large sample of protocluster candidates at z∼3−5 was constructed over the 25 deg2 DUD layer, and follow-up spectroscopy confirmed a protocluster at z=3.70. Here, we propose further follow-up spectroscopy to completely map out the cosmic web around the protocluster. The target region is fully covered by allocated JWST/NIRCam observations, opening a window on the stellar population and structural properties of z∼4 galaxies in and around the protocluster. Exploiting the wide FoV of DEIMOS, direct probes of escaping Lyα emission will for the first time be contrasted to rest-optical sizes and morphologies, tracing the stellar distribution. The spectroscopic redshifts will securely separate foreground/background interlopers at the same general epoch within the first 1-2 billion years of cosmic history from genuine protocluster members. The combination of spectroscopy and deep NIR imaging will sensitively improve constraints on stellar population, which will be compared between field and overdense environments, as a function of 3D location within the protocluster, and between dropout galaxies with and without Lyα emission.
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