Originally Posted by jagans
I would never go from larger to smaller on any type of drainage system.
Typically, yes but UPC 316.4.2 "No fitting or connection that offers abnormal obstruction to flow shall be used. The enlargement of a 3" closet bend or stub to 4" shall not be considered an obstruction
This comes in real handy when doing a ground rough for a slab.
By stubbing up 4" you put the flange into the pipe so you don't need to put excessive pipe wraps on the exterior of the stub to receive an outside hub.
These are the same pipe wraps that the concrete finishers either raise, lower or destroy- causing you to chip the concrete away from the stub to receive the hub