Stahl confirmed this week in an interview that neither he, nor his co-star were going to be part of the project.
"None of the cast is coming back." Stahl said. "The [T4] story is changing conceptually. I believe it’s a jump to the future, so my character will be quite a bit older. That’s all that I know. So I’m not coming back, which is a drag."
Stahl equally mentioned that the producers did not necessarily foresee giving a part to Arnold Schwarzenegger, who played in all three preceding films.