The focus and plot of this film deviates from the "premise" established by the first three movies. We are given a larger world view of John Connor's life after Judgment Day. The focus is more on the human Resistance against SkyNet, rather than showcasing the tension of a killer machine hunting down its target with the sole intention of terminating that person. This movie loses a lot by deviating from that established premise.
John Connor comes off as a supporting character here, the focus being more on Marcus Wright and Kyle Reese. There is no build-up to a climactic showdown between the terminator and its target, which was the strength of the original three films.