"How expensive would it be to replace 4 4" and 2 6" wood posts with steel on a typical deck?"
Wayne, Thanks for your inquiry. Assuming the existing footings are reusable and we are not dealing with a deck that is exceptionally high (let's say, under 15 feet above the exterior soil grade) I expect the costs for each new post to be in the range of $350. If you need new footings as well, you should expect more like $680 each. You really need to make sure that the base of the steel posts are above the exterior soil level so that you don't have future rust problems. Give us a call to schedule a site visit if you need help sizing the posts or evaluating the existing footings. Hope that helps!