The market price of insecticide is initially $10 per unit. To address a negative externality in this market, the government decides to charge producers of insecticide for the privilege of polluting during the production process. A fee that fully takes into account the social costs of pollution is determined, and once it is put into effect, the market supply curve for insecticide shifts upward by $4 per unit. The market price of insecticide also increases, to $12 per unit. What fee is the government charging insecticide manufacturers?