The Duraline HDPE Innerduct, 4 in, SDR 13.5, 500 ft, Smooth, Black (vendor part 2042331210, Millennium part 521423), provides a smooth-wall path inside larger conduit for pulling or jetting fiber cable with less friction than corrugated duct. The SDR 13.5 wall thickness stands up to standard direct-bury and bore installations on OSP fiber builds, and the 500-foot reel length matches common crew handling. Conduit installers use it to subdivide larger duct into multiple cable paths. Stock it with matching couplers and duct plugs to complete 4-inch duct runs.