This is because the z coordinate is -4. So we move down 4 units on the z axis. The z axis is basically a vertical pole planted in the ground, and the xy plane is the flat floor.
It's a bit tricky to visualize so try to imagine that you're in this 3D space personally.
Or you could imagine that something like z = -4 means you are in basement level 4 of a building (say a parking garage) and something like z = 6 means you're on the 6th floor above the ground.
