In Terminator movies 1 to 3 Skynet sends one Terminator at a time and The Resistance sends one defender at a time. This can be easily explained with time travel being resource consuming.
Here's what happens in Terminator 4.
Skynet stages a complex scenario to fool John Connor and make him go into one of Skynet facilities. That facility among other functions produces Terminators - we see recognizable parts on the conveyors. So in fact John Connor gets into a territory that is fully controlled by Skynet and is likely stuffed with Terminator parts and fully assembled Terminators. Yet when Skynet wants to kill John Connor it sends a single naked Terminator without any weapons again and that Terminator wounds John Connor with some metal rod.
Is there any reason for this except to have drama and action in the movie?