Producer Mario Kassar recently spoke to a Lebanese film magazine regarding production for Terminator 4 and TheArnoldFans transcribed a copy of the key elements of the article:

"T4 is in the works. The screenplay will be ready in 8 or 9 months. Once we have that, we will proceed with the production. The question that has never been answered in all the Terminator films is time travel. You can come from the future to change the present. But we never see how they go back to the future, which is a subject that can be explored. I think you already know where T4 is going from the ending of T3".