If there is a T4, god forbid movie studios don't ruin a good thing, it will more than likely have to be Arnold-less.
Why? He said his mission was to insure that John Connor and the misses make it out alive to start the resistance, which they did. T-850 was destroyed in the process. If he were to return in a T4, his mission in T3 would be obsolete. Not only that, he'll have to return for a FOURTH time..and be mission less..which makes no sense. On top of THAT, whatever model they throw him into for his 4th appearance will be obsolete, especially since the machines apparently keep evolving, (850, 1000, TX)
I think that although it was left wide open, there was also a good bit of closure. Everything that was forseen, happened. Judgment Day, albeit delayed, did begin. Now, J.C. is going to lead the humans into battle against the machines, and there ya go.
T4 would more than likely be Arnold-less, like I was saying, because he is dead for one. Secondly, there would have had to been 2 Terminators that the resistance reprogrammed to save/help John Connor, which is not likely. Therefore, there is no reason why a machine would be helping the other side when they are the ones trying to defeat them.
Arnold is 55 years old. He already said he wanted to get into politics. Besides, they will more than likely have to have as big a gap in between a T3 and T4 as they did between T2 and T3. 12 years? Arnie will be 67, and NOT doing his own stunts. And as I said, there wouldn't be any need for a Terminator in T4.