I personally do not get throttling a 4 inch (It looks like PVC to me, Not ABS. ABS is usually black) pipe down to a 3 inch toilet flange. The mason should have wrapped the pipe with foam, or cut out a hole in a piece of Bead board and put it over the pipe to give the plumber clearance to properly set the flange, if you ask me. Non shrink grout could be placed afterward. I think thats what Ben means, why lose the benefit of the 4 inch pipes diameter.
Its like putting in a 4 lane highway, and bottle necking it down to 3 lanes. Picture the cars as turds.
" A lot of men build things, and a lot of things fall down "