4x4 hub donut. Cast-iron (soil pipe) hub adapters. The cast-iron hub adapter series will connect plastic DWV Schedule 40 or Schedule 30 piping directly into the hub of cast-iron (soil) pipe without packing, chemical mixtures, or other materials. The resultant joint is water and gas tight. The fittings listed are for service weight cast-iron hubs only. 4" x 4"