You are right in saying that to bring Roland Kickinger in T4 just because he looks like Arnold would turn him into a man who lives in the shadow or name of Arnold and is famous because of that but not the original him.
Kind of like how Brandon Lee faced challenges of making a name for himself but not the next Bruce Lee or living in that shadow or famous because of Bruce Lee. And Brandon Lee achieved that status.
The reason I mentioned Kickinger because if Arnold Schwarzenegger is important, then to make that character gone would make Terminator fans feel the loss of Arnold. It's kind of like watching a Superhero movies like Superman, Spider-man and Batman with the same Hero / actor for several movies and suddenly a new actor is playing the Super-hero character. And the fans would not be able to connect to that new character because they have grown with the particular character / actor.
It would be like watching a different movie and because of that a movie can flop. So if Kickinger is the pick, the fans can feel like watching Arnold ( young Arnold ) back in action, they can still connect to it which is why I stated him. Unless of course the story takes a different turn, evolves into something new and different which accordingly changes the characters. The Rock would be great no doubt about that. The Terminator that Arnold was, would never be forgotten..especially the one in Terminator 2.