Terminator #1: Robots go back in time to kill the mom
Terminator #2: After not only failing to kill the mom, but actually prompting the birth of the person they wanted to kill the robot's wisen up and attempt to kill the kid. Why not go to a point when the kid was an infant to avoid all the fukery remains a mystery.
Terminator #3: After botching both of their previously ill contrived plans the robots decide against striking at the Teflon Don and instead go after his lieutenants...which in and of itself is as ignorant a plan as possible.
Terminator #4: Nothing really happens in terms of time travel and the overall plot of the war isn't really moved at all. A base is destroyed, some Terminators get blown up and a lot of supporting actors die, ultimately though the movies serves no purpose
Terminator Gen: The machine sends a terminator back in time prior to the first time the machines had already sent a terminator back in time...which may actually be the most common sense thing to do, it appears as though the time line changes making Terminators #1-4 null and void.
IMHO the franchise should go all in and send terminators back in time to historically relevant dates in order to kill the Connor bloodline. I for one would not be opposed to a Terminator vs. Abe Lincoln the Vampire Hunter, who in a twist turns out to be the great, great, great, great grandfather of John.