While getting estimates for spray foaming my attic, the sales person stated that research has determined that you do not need more than 4 inches of spray foam insulation. He stated the heat loss/transfer was negligible after 4 inches. He stated that z4 inches would stop 95% of the heat transfer. He stated to go from 4 inches to 5 inches was not worth the money. I live in Delaware where the code requires R38 in the ceiling. If 4 inches is approximately R26, how can I only use 4 inches of spray foam insulation? Is the contractor correct?