The idea of a reboot is pretty dull to begin with and bringing Arnold back for it would make no sense.
Arnold could be carved out of solid rock and in the best shape of his life â€“ he's still north of 60. Practically, would a brilliant computer overlord send a 60-something looking cyborg to terminate an enemy? Or would it send a cyborg that looks like, well, Arnold, circa-1984?
Of course, you could argue that realistically a Terminator would be more effective if it blended in with the crowd. But then why go with a guy who (even at his age) looks like Arnold Schwarzenegger? Why not a completely innocuous looking person like, I don't know, Paul Giamatti?
A reboot with Arnold just sounds like all sorts of ridiculous.
Even if the movie didn't work, I personally liked where T4 was going with the franchise. Tell a different story with new characters in each, but set it in the post-apocalyptic future with John Connor as a background character. Hell, have Arnold in one as the human template for what eventually becomes The T-800, as was suggested in T3.
Monster Zero's "Most Naive Person About 'The Business'" for 7 years running!