Apparently the governator won't be returning for the fourth entry in "The Terminator" series. Conveniently its because his type of killing machine hasn't been designed yet.

From Arrow In The Head:
A couple weeks ago, Ammon told you about some of the more in depth details on the plot for TERMINATOR SALVATION: THE FUTURE BEGINS. Well producer Moritz Borman spilled some interesting details recently!

Interesting indeed. Apparently Arnold Schwarzenegger is a definate no-go for the new film. According to Borman:

“I can tell you this much — this is the story everybody has been waiting for. Namely, we’re in the future. The part of it where Arnold’s character has not yet been built but is on the drawing board."

But Borman doesn't rule out the chance for The Govenator to star in the other two films of the trilogy.

“It’s convenient that he hasn’t been built at this point of the story, as Arnold has more important things to do right now, but that doesn’t mean he might not re-appear later in the trilogy. He knows the material and is very supportive of what we’re doing. There might even be hints of his character in this one.”

Now that's just exciting! Not too long ago there were rumors swirling around the net about Arnold's acting career getting resurrected after his tenure as Governor, which assumingly falls in line with the production of the second part of this new TERMINATOR trilogy. As for Vin Diesel being in this one, Borman says:

"I can tell you that Vin Diesel is not somebody we’re currently talking to.”

Source Arrow In The Head