You should see the first three movies! The first (The Terminator), the second (Terminator 2: Judgment Day) and the fourth (Terminator Salvation) ones are my favorite for both the storylines, the Sci-Fi tech and the action! The third movie (Terminator: War of The Machines) was a bummer since the actor who portrayed John Conner made the character look like a bumbling weakling in contrast to the superb, strong leader of humanity in the future (I think that the writers were trying to show the more human side of John Conner with all of his faults). Also, it sucked because Judgment day still happened (But that was already a given).
But still you have to see it because it officially (For Now*) bridges the story between the second and the fourth movies. (* - Since the reboot movie, Terminator Genisys, was released which changed the timeline, fans are not exactly sure what should be considered canon in the franchise anymore - There was talk about Arnold Schwarzenegger getting together again with former co-star Linda Hamilton [She played Sarah Conner in the first two movies] to create a new sixth movie, Terminator: Dark fate, that would reveal another version of the events that happened after the second movie [No details about the movie's story was leaked out to date] ->
https://en.wikipedia.org/wiki/Terminator:_Dark_Fate, while ignoring the timeline in the third and fifth movies (The fourth movie, Terminator Salvation, could stand on its own as a separate Sci-Fi movie since the familiar "Arnold" Terminator did not play a major role in that movie's story [It's just a terminator of the same T-800 series and Model 101 "Arnold/SGT. John Candy" skin]. Its official release date is now supposed to be November 01, 2019 [It's changed before, so who knows if it'll change again?]).
