I think you mean "styrofoam". For something 4'x8' you might want to try one of the big box hardware stores, you'll probably have to sandwich a few together to get that thickness. If you do that, wood glue works nicely.
Thickest he may be able to find is an R-10 (depends on what part of the country you live in) with is 2" thick. He'll just have to glue a few sheets together.