Just a fun mind **** to get away from the RoboCop and Blade Runner reboot and sequel threads...
Terminator: Salvation is really a prequel, isn't it? Because John hasn't sent Reese back in time yet, so the first movie hasn't occured.
BUT, John is alive, which means that Reese had already gone back in time and fathered him, making it a sequel.
BUT, we don't have the T-800 series yet, which we see in the first one, making it a prequel.
BUT, it takes place years after the last installment, making it a sequel.