Here is some more info from Moviehole about casting for T4:
"Anyone wanna know who the previously announced Common is playing in the new “Terminator” movie?
No, not Sarah Connor – though they have the same muscle definition.
According to via JoBlo.com, the rapper cum actor will be playing the older version of a character we have met before - Daniel Dyson, son of Miles Dyson, the guy who accidentally contributed to the creation of SkyNet and ultimately sacrificed himself at the end of “Terminator 2 : Judgement Day” (I believe the character of Daniel also briefly turned up in episodes of “The Sarah Connor Chronicles” this year).
Aside from John Connor (and if you count the 'Director's Cut' of "Terminator 2", Kyle Reese), Dyson will be the only character from the second "Terminator" movie to return for the new McG-directed "Terminator" movie. Both are played by different actors (Christian Bale is Connor)."