4″ Filter Replacement Window. These Windows are 1/2” thick Polycarbonate that fits all 4” Hi-Flow, Dry-Sump and Screw-On See Through Filter Assemblies. The windows are offered with one small difference, the thickness. The poly carbonate sheets that are used come with a tolerance of +/ 0.010. The windows come either 0.475″ thick, 0.480″ thick, or 0.485″ thick. You can determine which window is needed for your filter by looking through the window either towards the center of a 4″ filter or near the inlet ports of a 6″ filter for a number 475, 480, 485, or 490 referencing the window retainer used for the correct thickness of window. Plastic Material: Polycarbonate Plastic Finish: AR2® coating Plastic Thickness: varies between 0.475″ to 0.485” Quantity: Sold Individually Notes: Can only be used with 4” See Through Filter Assemblies