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We present the results of a survey of the brightest UV-selected galaxies inprotoclusters. These proto-brightest cluster galaxy (proto-BCG) candidates aredrawn from 179 overdense regions of $g$-dropout galaxies at $z\sim4$ from theHyper Suprime-Cam Subaru Strategic Program identified previously as goodprotocluster candidates. This study is the first to extend the systematic studyof the progenitors of BCGs from $z\sim2$ to $z\sim4$. We carefully removepossible contaminants from foreground galaxies and, for each structure, weselect the brightest galaxy that is at least 1 mag brighter than the fifthbrightest galaxy. We select 63 proto-BCG candidates and compare theirproperties with those of galaxies in the field and those of other galaxies inoverdense structures. The proto-BCG candidates and their surrounding galaxieshave different rest-UV color $(i - z)$ distributions to field galaxies andother galaxies in protoclusters that do not host proto-BCGs. In addition,galaxies surrounding proto-BCGs are brighter than those in protoclusterswithout proto-BCGs. The image stacking analysis reveals that the averageeffective radius of proto-BCGs is $\sim28\%$ larger than that of fieldgalaxies. The $i-z$ color differences suggest that proto-BCGs and theirsurrounding galaxies are dustier than other galaxies at $z\sim4$. These resultssuggest that specific environmental effects or assembly biasses have alreadyemerged in some protoclusters as early as $z \sim 4$, and we suggest thatproto-BCGs have different star formation histories than other galaxies in thesame epoch.

The brightest UV-selected galaxies in protoclusters at $z\sim4$: Ancestors of Brightest Cluster Galaxies?