how far would rims with 4.5" back spacing stick out?

 

It depends how wide the rim is. We need more info that what you provided. What model and year of vehicle do you own. What rim size do you have on it now? What size are you planning to run?

Later, Doug

 

its a 97 blazer ls 4x4, i just ordered some pro comp wheels, that are
15x7 with 4.5" back spacing

 

From what I've found the stock 4x4 wheel specs are 15x7.5 and 5 7/8 backspacing. So they rim will stick out ~7/8" further than the stock rim IF the specs I listed were correct.

Later, Doug