Use to convert your system to 3" pipe at the floor for low flow situations when high suction fans are required. The 4" piece of pipe goes into the floor and the lip of 3x4 reducer will rest on the floor if you have a 4.5" hole in the floor.
Or use with 4x6 Floor Bushing to reduce to 3" PVC. Shown in Picture 2 used together.
4" length of 4" PVC included to connect bushing and reducer.