What is the price per unit that producers receive after the subsidy?

The original supply curve is S, and the demand curve is D. The subsidy shifts the supply curve to S1. The new equilibrium is the intersection of S1 and D.
From the diagram, the new equilibrium occurs at a quantity of 8 (000 tons) and a price of $2.

At the new equilibrium, consumers pay a price of $2 per unit. The total subsidy per unit is the vertical distance between the original supply curve (S) and the new supply curve (S1) at the new equilibrium quantity of 8 (000 tons). At a quantity of 8 (000 tons), the price on the original supply curve S is$4. At a quantity of 8 (000 tons), the price on the new supply curve S1 is 2. The subsidy per unit is the difference between these two prices: $$ \text{Subsidy per unit} = \4 - $2 = $2 $$

Producers receive the price paid by consumers plus the per-unit subsidy.

$\text{Price producers receive} = \$2 + \$2 = \$4$

The price per unit that producers receive after the subsidy is \boxed{\mathbf{\4}}$.