Backspacing is exactly what it says - the distance from the edge of the rim to the back of the hub mounting surface.
Offset is either positive or negative if you have any. The thing you haveto keep in mind with offset is that you have to realise that the wheel is actually a full inch wider than stated specs when figuring out backspacing.
Positive offset will move the hub surface toward the outside edge of the wheel and negative toward the inside edge.
In your case, with 15x7 wheels, which are 8" across - you have 0 offset and a backspacing of 4 inches - see how what I said works out?
15x10 wheels woould actually be 11" wide, so with 0 offset would have a backspacing of 5.5", but you have a negative offset (which moves the hub mounting surface toward the inner rim of the wheel of 45mm, which is a little over 1.75" and you wind up with 3 and 111/16" backspacing - which will fit strutwise, but not in stock fenders