The makers of Terminator 4 have released a press release, which says that filming started earlier this month. We knew that already, but they also have included a plot synopsis which confirms what we knew, but also has some new stuff in as well.

The press release for Terminator 4 through Movies Online, sounds something like this:

Set in post-apocalyptic 2018, Christian Bale stars as John Connor, the man fated to lead the human resistance against Skynet and its army of Terminators. But the future Connor was raised to believe in is altered in part by the appearance of Marcus Wright (Sam Worthington), a stranger whose last memory is of being on death row. Connor must decide whether Marcus has been sent from the future, or rescued from the past. As Skynet prepares its final onslaught, Connor and Marcus both embark on an odyssey that takes them into the heart of Skynet’s operations, where they uncover the terrible secret behind the possible annihilation of mankind.

The film also stars Anton Yelchin as Kyle Reese, Charlotte Gainsbourg as Kate Connor, Moon Bloodgood as Blair Williams, Common as Barnes, and Jadagrace as Star.

So that kind of explains Sam Worthington's part in the film, and there are a couple of new cast members in there. Charlotte Gainsbourg we know, but Jadagrace not so much. In fact I can't find anything on them. The first film of the new trilogy is due to be released in the Summer of 2009.