I love this show which I initially thought would fail. My only problem is that with with the future having been seen and created on one level, that there is now no way to change the past since the future can always go back and change the events that could destroy humanity. We are beginning to see this take effect. Plus the idea of multiple futures is going to create multiple pasts thus multiple timelines making the future very confusing on both the humans and machines. Not sure what show (maybe History Channel or Discovery) that discussed this and the actual chance that this is possible. Really quite confusing if you are not an physic or astrophysic.
The entire show is an alternate timeline, at least from the T2 to T3 standpoint. T4 is going to take place based on the events in T3. SCC started between T2 and T3, and thus wiped out the T3 timeline when they jumped to the future.
Actually, it's my understanding that T3 may have zero impact on T4, since it's essentially a prequel to T1 (despite being in the future). T4 takes place before anyone is ever sent back in time, so it would fit with chronology of T1 or the series, assuming the series does not avert Skynet from taking over.
I like the idea that the future is constantly changing as present events are changing...