My wheels are 15x8, with 4.5" backspacing. With the 32x11.5-15 im running, and 2.5" lift, I had to bend up the fender lip to keep them from rubbing. Even with that, I cant even squeeze my pinky finger in between the tire and fender, when its stuffed...And thats considering it stuffed one at a time, and so the tire is angled up into it. If I hit a large drainage ditch straight on at say, 30mph+ ...they do contact a bit. So, I dont think 4" backspace would be enough.
Got pics in my gallery of it stuffed...