Here's an excerpt on Breaking Dawn from the the actor's SciFiWire.com interview highlighted today on Twilight Treasury:
"Marcus also plays an important role in the fourth Twilight novel, although Heyerdahl hasn't peeked that far ahead yet. "I have left that book alone," he confirms. "I want to leave it until we get closer to the movie. I was talking to Stephenie about it, and she asked if I had read the final book. I said 'Nope, I don't want to read it yet. I want to leave it alone until we get there.' 'Oh, just you wait. You'll love it.' I do know generally what happens, since it is impossible not to find out. And, yes, I've been contracted for the final chapter."
Sounds like Christopher is ready to sink his teeth into the final Volturi stand (pun intended) - and I'm likin' that!