Biased galaxy-formation theories predict that massive galaxies at high redshifts should act as signposts to high-density environments in the early universe, which subsequently evolve into the cores of the richest clusters seen at the present day. These regions are characterized by overdensities of young galaxies, perhaps including a population of dusty, interaction-driven starbursts-the progenitors of massive cluster ellipticals. By searching for this population at submillimeter (submm) wavelengths we can therefore test both galaxy- and structure-formation models. We have undertaken such a search in the field of a z = 3.8 radio galaxy, 4C 41.17, with the SCUBA submm camera. Our extremely deep 450 and 850 mum maps reveal an order-of-magnitude overdensity of luminous submm galaxies compared to typical fields (the likelihood of finding such an overdensity in a random field is less than 2 x 10(-3)). The SCUBA galaxies have bolometric luminosities of more than 10(13) L-., which imply star formation rates (SFRs) consistent with those required to form a massive galaxy in only a few 10(8) yr. We also note that this held exhibits an overdensity of extremely red objects (EROs), some of which may be associated with the submm sources, and Lyman-break galaxies. We propose that the overdensities of both submm and ERO sources in this field represent young dusty, starburst galaxies forming within a protocluster centered on the radio galaxy at z = 3.8, which is also traced by a less-obscured population of Lyman-break galaxies.