One thing though - John Connor must know about the time paradox as he was witness to the conversation about how Cyberdyne based all its work on the original Terminator, and he knows his own dad was from the future.
But why doesn't Skynet become aware of this paradox, and therefore realise that it must send its Terminator to 1984 just to ensure its own existance in the first place?
...But what was to stop Skynet from quickly learning after it became self aware that its own inception derived from mysteriously 'found' technology that was far beyond what was otherwise possible as of 1984? Did it simply not put 2 and 2 together here?
Without T2, John nor Sarah would have known how Skynet started. To them, as it was to us until T2, it just would have been something that happened regardless of events in '84. We didn't even know it was a Cybercynde building at the end of T1 until T2 because the scene where they find the chip (which was filmed for T1) was deleted.
Now if you throw in T2, yes, they find out from Dyson how Skynet will come from technology developed from the Endo remains, but it will not be until they blow up the new Cyberdyne building that the timeline of J-Day and the War Against the Machines will scew into an alternate future. Technically, the timeline scewed as soon as the T-1000 arrived, but that's because his actions will lead to the destruction of that building.
However, John would know of the paradox with his father regardless of timeline just because that is the core of his existance, but he would still only know what has already happened, and that from Kyle's account to Sarah.
The scene in T4 with him confused over Sarah's tapes is great because she is giving him info based on Kyle's account of a different timeline, but what is happening in the current movie timeline isn't what once happened, which again proves that there must be at least two timelines in order to accept any and all of the sequels.[/QUOTE]