The cast aren't yet signed, they don't know if it's going to be one film or two, the script isn't finished and they haven't figured out the effects, but The Twilight Saga: Breaking Dawn is expected nevertheless to start shooting this coming autumn, shortly after the release of third instalment Eclipse. That's according to series producer Wyck Godfrey, talking to the LA Times, anyway.
The fourth book in the series, Breaking Dawn presents some extreme filmmaking challenges, a few of which are neatly summed up here in case you're spoiler averse.
SPOILERS There are also some effects challenges presented beyond the body-horror aspects listed in the link above; the second half of the book features a half-human, half-vampire infant who is walking and talking by the time she's about a week old, requiring massive amounts of CG either in conjunction with or instead of an actual baby actor.END SPOILERS
With the script and treatment still not settled, this is all very much up in the air, but the grosses of the last two films suggest that it will get made. If they can just figure out how to get round the gross bits...