More than likely you will need a 18'' slab in NY, a 4'' is more like for patios, not for foundations for building structures. Also given the weather in that area you are going to need something that is going to stand up to the cold and snow.

I personally would start by having some people come out and quote me for a new foundation, because you ain't going to do that yourself, it's too much concrete, and then you will get some professional ideas of options you have.