Backspacing confusion

Example:
My wheels are 4.5 backspace and I have a gap between the tires and the quarter panel of 1 inch, and a gap between the tires and leaf springs of a half inch.
If I wanted wider tires I would have to get a 4 inch backspace to move the rim and tire a half inch farther out so they would fit in the opening without hitting the springs or the fenders.