Director McG gave an interview (here) to Film Journal International (it's a trade publication and website covering the motion picture industry, so we may consider it a reliable source). An he's been surprisingly talkative about what could be ahead with the movie Terminator 5:
Don't really really know yet if he's half-joking or speaking the truth, but such a plot would be awesome! I almost wet my pants at thinking about it! Bring it on McG!
The fourth movie isn't yet out that I'm already excited about Terminator 5! Love Hollywood!