The 4.75" backspace come closer to tucking than the 4" will. I have 15x8 with 4.75" backspacing with a 275 on my 99 and it's about flush with the fender. I'm lowered roughly 3" in the rear. The 4" backspacing would push the wheel out further.

 

Bogart makes a 15x10" on a 6.5" backspacing. Does just fine on a stock rear end and stock leaf pack truck.

 

I'm lowered 5" in the rear and I tuck 295/65/15 with a 10" wheel and 6.5 bs