To help visualize what is going on with the wiring.
This is how most programs would draw it out, but it’s not that helpful.
This is what it would look like in real life and how the wires would connect.
Now take note, even though you are reversing the wiring on one of the drivers so that as one moves forward, the complementary one moves backward, and vice-versa, they are still in Parallel. That make the two 4 ohm drivers equal to 2 ohms. Then you do the same with the second set and have another 2 ohm pair. Now these pairs get wired together in Series making the total circuit go back to 4 ohms.
Hope this clears things up.