I would like to change all 4 wheels,Right now my rear wheels are 15 by15 with 4 1/2 back space. my ? is if I go somthing like 18 by 12 wheels in the back .will my back space still be 4 1/2 with the new wheels.

Depends on the tire also. A freind of mine went from MT Sportsman 33-21.5 to the new MT S/R Radial 33-22.0 and they are at least 2 inches wider overall than the Sportsmans. I don't know what tire you have, but it seems to me if you run a wheel that is 3 inches narrower than you have right now, but with the same bs, your wheel will be to far under the car and not look right.

You could run a wheel with a 3" backspace. That would put the outer edge of the tire in the same place it is now if the profile of the tire is the same. With the 4 1/2" backspace you would have 1 1/2" more clearance between the fender lip and the tire.