Pool Filter Replaces Unicel 4CH-21, Pleatco FC-0121, Filbur PTL18P-4 Filter Cartridge for Swimming Pool and Spa Replacement filter cartridge for unicel 4ch-21, filbur ptl18p-4 and pleatco ptl18p-4 Replaces pleatco fc-0121 Replaces filbur ptl18p-4