OK, first of all, T3 was released in the US before Australia (July 2 in the US, July 17 here). I just saw it with a friend.
This is what I think:
In T1, John Conner is probably the child of ANOTHER man. He then becomes the leader of the human resistance in the future when the machines take over. Finding out the machines have sent a Terminator (T1 Arnie) back through time to kill his mother, Kyle Reese, being John's best soldier, is also sent back through time, to protect Sarah. He then becomes John's NEW father, and John is now brought up
knowing he will lead the humans.
SkyNet is a virus-destroying program that has it's own AI (as seen in T3). I thought, from T1 and T2, that it was a company that creates AI for machines. I guess it isn't.
In T1 and T2, Judgment Day comes in August 1997. The chip and arm (found from T1 in the factory) Miles Dyson uses for his research speeds it up, but J-Day still comes in 1997. Because Sarah, John and "T2 Arnie" destroy CyberDyne, research is set back, and the new date for Judgment Day is whenever T3 is set (2003?).
In T3, John says that he meets Catherine's father the day after they make out in that basement, and would therefore know about the Shelter and how to fight the machines. In T2, "T2 Arnie" comes back in time, and 'cancels' that meeting, and the new meeting date is in T3 at the Military Compound. John and Catherine would have gone into the Fallout Shelter at Crystal Peak anyway and helped the humans anyway.
The T-X is sent back through time to kill John Conner. Her side missions in getting to Conner is to kill his lieutenents, so if she fails to kill COnner, he will at least have no lieutenents to help him. After John is terminated by "T3 Arnie" in the future, "T3 Arnie" is captured by the humans, and is programmed by Catherine to protect John in the past.
There is no way to stop Judgement Day. It is the military's obsession with created AI defence systems that makes it happen, If research is interrupted, D-Day will only come at a later date.
In other words, out future is f**ked up
The creation of a T4 movie will only probably be based on the humans fighting the machines, and winning, but that is something we don't need to see.
If they make it around the humans winning the war, it will have to show Conner/Catherine sending Reese, "T2 Arnie", and "T3 Arnie" back through time.
T4 would therefore be one confusing mess, unless they base the war one some alternate dimension where none of the events of the past three movies ever happens.