So, T4 picks up in 2018, post Judgement Day. Christian Bale IS John Connor. Warner Bros released this synopsis:
Set in post-apocalyptic 2018, John Connor (Bale) is 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.
Kyle Reese also reappears in T4. I don't think Arnold has signed on, but there is a rumor that Josh Brolin will play a terminator. Charlotte Gainsbourg plays Kate Connor (nee Brewster.)
I am excited. They just started shooting.

Check out the Terminator website updated by the director, McG (of Charlie's Angels.)