First of all how many fixtures is that 4" vent carrying total? Is this the only vent coming up through your roof? I imagine that vent is carrying a toilet,sink shower and tub right? If thats all its venting then yeah I'd say reduce the pipe to 2". so you will have a toilet,tub and sink venting out a 2" line. That will be fine.
If you cant do this due to more fixtures through out the house being tied into
this 4" vent I'd say its time to fer the wall out to make room for your vent and say good bye to 31" of toilet space!!!!!!!!!!!!!!!LOL. Is this being inspected?