McG dropped another hint that would have us believing that The Governator himself is going to show up in T4 in one form or another. I've always felt that it would make sense to just scan him in and use his digital likeness to represent the T-800. That way he would only have to be there a day or two during the whole production run and he could stay governor. But apparently McG wants the younger Arnold in the movie. But how...?
From The Playlist:

"The T800s as we know them look like Arnold [Schwarzenneger]. The T800 plays a big role in this movie.. I can't quite tell you how we express that, but I will tell you the following: I think when you make a Terminator movie the responsibility of our team...is to push special effects forward. So what we're trying to do is create visual effects and write code that's never been written before to address this very question. It is not clear if we will be successful or not, so we're working very diligently on going up to ILM in Sn Francisco because soon to address this very nature because... let me just hint at this: I don't want today's Arnold Schawzenneger. Cheers [to him], but I just want idealized Griffith park, Bill Paxton, tire-track on a face on-the-face archetype, and we'll see what we can do about that."

So maybe Arnie won't actually be in the movie but it sounds like his digital doppelganger will be.