Oh that screws me because I was planning to redo my upstairs bathroom that had been tied into a 4" vertical stack that continues up thru the roof.
It has a cast iron long turn combo wye laying flat on its side, with the toilet, lav, tub, and kitchen, all coming into the long turn end, then out the end of the combo a street 90 down into the stack, and off other end, another street 90 up the vent.
I was going to redo the cast iron in 4" PVC with the toilet flange in 4" dumping into one side of a 4" vertical double wye, 4" vent in center up to the roof, and then dump the lav, tub, kitchen into the other side of the wye but with a 3" bushing.
My thinking was the toilet would flow down a wye much better than a double sani tee without splashing across and get enough vent air since its 4".
With the other drains dumping from a 3" pipe into a 4" wye, with 4" vent in center, I thought that was plenty of air space, and the wye made all the drains at the same height so no wet venting is above the wye.
The kitchen line coming in will have a separate vent, but the bathroom lav and tub is not vented due to its so close to the stack and the wye I was going to drop into.
So I guess that would not be code compliant using a vertical wye with vent in center, but a double sani tee would be?