Wheel backspacing

4" bs spacing would not be good. If you want the wheel to be centered like it is now you would just add 1" of backspacing to what your running now.
Your 8" wheel has 4.5" get 10" with 5.5" backspacing. I like the ideal of using the spacer to fine tune wheel fit if necessary. That is far better than not having enough backspacing. IMO