It's a sequel, because it's set in an alternate timeline to Terminator 1. The Kyle Reese from Terminator 1 comes back from the year 2029, from a timeline in which Judgment Day happens in 1997. In Terminator 2, they avert that future. In Terminator 3, Skynet kicks off a new war, with Judgment Day happening in 2008 (or whenever the movie was set). Terminator 4 follows on from that, so even though there's a Kyle Reese in it, he's not the same Reese from Terminator 1.
Just another reason why this film will be shit.