Do Wheel Spacers Increase Backspacing?

Spacers give you less backspacing and more offset. Backspacing is the difference between the hub mount and the inner rim of the wheel.

Do wheel spacers change backspacing?

Spacers can only lower the backspace of the wheel, so you're right on your assumption. It's moving the mating surface of the wheel (at the hub) closer to the backside of the rim by the thickness of the spacer. It's also moving the outer face of the wheel, so keep that in mind.

What offset is 4.5 backspacing?

For example, an 8 inch wide wheel with a 4.5 inch backspace would be the equivalent of a zero offset because the mounting surface aligns with the centerline of the wheel.