Backspace for front wheels... Only 4 inches..

Can someone advise me what my front wheels should be when referring to backspace need.
I have the 4.5in. for the rear rims...15 x 8 with 4.5 inch backspace.
I just need to know what size I will need it to buy when it come to the front rims. 3.5 (my case, it's a 4 inch rim) what should the size be for the backspace ?
This is a stock front end with just the front brakes being swapped/changed for discs.
The front are now discs and the rear are drums.
Thank you (All) for the answers, in advance.
MAPS

P.S. Just that I need to know (yesterday) as I'm buying them, today !