This is a remodel of a 2nd floor bathroom. Original plumbing is hubbed cast iron and screwed steel. Mods will be made in PVC, with tie-in by screw and/or no-hub compression couplings.
Looking at the wet wall from left to right I have the 4" iron main stack, existing sink drain, 3" screw pipe vent for downstairs toilet. Vent is commoned back to the main stack at about 4" height.
The sink drain currently runs laterally into the main stack. The sink drain location is moving to the right of the 3" vent. Should I consider draining the sink into the 3" downstairs toilet vent? Or should I add elbows to the 3" vent and dog-leg it around under the new sink drain location? I understand a wet vent spanning floors would not be to code, probably for good reason. How bad is adding two, or possibly four elbows to bend the 3" toilet vent around the new drain location? Is 4 elbows in the vent ok? It would simplify the cutting and splicing.