in each one we see an attempot of skynet killing john connor.
the first one is when skynet sent the terminator to kill sarah connor before giving birth to john connor.
therefore if she died the rebelion would never happened
the second one is about killing john in his childhood but thnkfully he survived
the third is about killing john connor in his adolecence and his future companion kathryn bruester
and the forth one is during the war as i said