I'm going to get crucified for saying this, but given the direction #4 will be going in, it won't matter to me if Arnold isn't in it, the story doesn't necessarily require him.
If it's going to be dealing with the war between man and Skynet, that means that it will mostly take place on battlefields where Terminators aren't required to be covered in skin since the jig is most definitely up at that point.
If there's a scene or two involving "infiltrator" Terminators, then Arnold may be needed, but I would think that Skynet is smart enough to realize that the humans know what that model looks like with skin and discontined it.
Just because it's a Terminator film does not automatically mean that Arnold is called for in the story. They should make this one cold, hard and gritty as hell, with an R rating if possible, I want to go into the theater and live the nightmare that Reese was talking about in the original, he had pure pain in his eyes while relating that whole thing to Sarah...I want that to come across in this one.
