Ok, here is a 3840×2160 display:
It has dimensions of 3840 wide by 2160 tall. That is a 4K display.
If we had two 4K displays, it would look like this:
Now, the width is twice as much, 3840 × 2 = 7680. But as you can see, the height is still the same, 2160, so the combined size is 7680×2160. So two 4K displays definitely does not give you 7680×4320.
If we stacked two more on top, we would have four 4K displays, like this:
Now, you can see both the width and the height have doubled, and we have 7680×4320, which is 8K. And as you can see, it is composed of four 4K displays.