Resod question

I just bought my first house a couple months ago, and I'm looking to hire someone to lay some new sod (something I've never done before), and was wondering if it's ok to just lay sod over a layer of topsoil. My house sits on a hill with an ~20% grade, and a lot of the soil seems to be eroding, so I want to try to stop the problem by laying new sod. The contractor I talked to today said that because there will need to be new topsoil thrown down (~4 inches), tearing up the old sod is a waste of time, and money. He said that if they just throw the new sod on top of the new topsoil, that will work fine. Does this sound right, or should I find someone else to do the job? Will this be enough room for the roots to grow? Thanks.
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