either back spacing is just fine. 4" will be sucked in 1/4" into the fender compared to the 3.75".
If you go with a 10" wheel, stick to 4" backspace or even 4.25". I've got 10's and 4" with 12.50s(though I'm double lifted) and have no issues at all. MANY members run 10's and 4" and even 3.625 or 3.75 bs with a bodylift and 33x12.50's. I see you not having any issues other than the almost required for all trimming of the valence in that bottom rear corner.
'01 XLT disguised as a '03 Fx4 Level II
Big. Loud. Slow. Bright.