IGN reports that a 5th movie is is going to happen with the return of the Governator (Schwarzenegger) himself.

http://movies.ign.com/articles/116/1164320p1.html

Huh. Didn't see that one coming, actually.

I'm glad a fifth movie is on its way but we have to wait and see what it'll be like, especially Arnold's role.

I'm curious to see what they have in plan for Arnold's character as well. I have a feeling its gonna be different that's for sure.

As far as I know. The film is not in production or already planned. Its just in the talks and Arnold won't be in the movie.

I do hope this doesn't spend much time in development hell. Last I read maybe a month ago, the rights to the film was still up in the air. I do want to see a conclusion to the series as much as I like it. Sadly, James Cameron I heard wouldn't direct because of his upcoming Avatar film. I'm kind of salty about that seeing as how it was supposed to be a new trilogy starting with Salvation.

Time will tell