Vent pipe question...
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If all fixture vents connect before exiting the roof, the vent seen from the roof top will be 4".
It depends not only on the number or types of fixtures, but length of horizontal piping as well. A 4" is not always required. Building code states that you must have at least one 3" vent to roof. This maybe different for residence. Residential codes are a lot more lenient.
The total vent area for a building is required to equal the area of the required building drain. For example, two 2" vents combined with one 1.5" vent is equal to one 3" vent, which would be required for most houses with 3 or fewer toilets. Hard to know the size of a vent through the roof without getting a close look from above.
You also don't need to have one vent you could have multiple vents in different locations, best way to plan it out would be a set of prints that coupled with the fact code rules depend on where the work is done. There is no simple answer.
