I just did 3.5" backspacing on 15x8's with aluminum drums. 4" backspacing probably would have been ideal from an aesthetic point of view but 3.5"s fit fine on my car and don't look out of place.
What size tire you planning on running? With the 4.5" backspacing you better go with a tire width that is for a 8" wide wheel or you could have rubbing on the frame.
Like Blackmetal said the tire is going to be tucked in and is not going to look good. I would go with the 3.5" backspacing if you plan on installing a wide tire, that will fill in the wheelwell nice.
I just re-read the post and noticed you were talking about convo pros. I have convo pros 15x8.5 /3.5" backspacing with 275/50/15 tires on the back with no rubbing. Stock 5379 springs and airbags that I just took out for the 5413 springs.