Ensuite dilemma

...........................

You definitely cannot "downgrade" a proper stack into a choked off version of it.

...........

Your dotted lines tell us NOTHING about how you actually intend to do the piping. As it is drawn, neither the toilet or sink have vents, so we would have to know WHAT you would be doing above the floor. The reason for the 3" vent is to comply with the regulation that the vent out through the roof hs to have the same area, not necessarily diameter, as the incoming sewer. (In your case, since you now have a 4" sewer, technically, the 3" is too small.)

This threat ain't going anywhere -- I'll look someplace else. thx for participating anyway