The beginning of 4 was actually the end of 3 and 4. Jigsaw dies in 3, but they dont find the tape until the autopsy. The detective who was sitting opposite of Wahlberg is the one who helped Jigsaw and Amanda, while Jigsaw was sick. They already knew there was another hand in the pot, because there was no way Amanda could have hoisted the Female cop onto that device. (where she was locked in and the key was dissolving in the glass of acid)
So yea, the Cop who listens to the tape is the next Jigsaw. But the whole "you thought you were done with the lessons" thing was directed at his apprentice.