According to Variety, Sam Worthington (Hart's War, MacBeth), having just completed his role in James Cameron's upcoming Sci Fi movie Avatar, was recommended by Cameron to McG, Terminator 4's director, for a staring role.

In Terminator 4, Worthington will be playing Marcus, who it's reported will be a main character over the upcoming Terminator trilogy.

The nature of Marcus' role in "Terminator: Salvation" is anyone's guess.
Terminator 4 will be set post machine/nuclear apocalypse with John Connor, played by the ever interesting Christian Bale, leading the good fight. However, given that rumors have Connor being more the leader-messiah than the main character of the series, perhaps the various headlines of "Worthington to star in Terminator" are on to something.

http://www.variety.com/article/VR1117980831.html?categoryid=13&cs=1&nid=2562
http://imdb.com/name/nm0941777/