The sentences that contain an example of contested usage are:
-
The postman needs to know who this letter is addressed to so it can be delivered. (The contested usage is "who...to," which should be "to whom" in formal English.)
-
Stephanie started reading the book first, and therefore she finished it before I did. (The contested usage is "therefore," which can be debated in terms of its correct placement and use.)
So the correct answers are sentences 1 and 4.