It doesn’t matter. You check the first of each group and pick the smallest, then compare the one you didn’t pick with the next one of the other group. In your example, you would pick all of the ones from the right side and once it is empty, just add all the ones on the left.
Wait, how do I know that all four of the right half aren’t smaller than all four of the Left half?
You don’t, and they can be.
Watch the animation on Wikipedia: https://en.wikipedia.org/wiki/Merge_sort
It doesn’t matter. You check the first of each group and pick the smallest, then compare the one you didn’t pick with the next one of the other group. In your example, you would pick all of the ones from the right side and once it is empty, just add all the ones on the left.