Solving the model, we get the following solution:

 Global optimal solution found.

 Objective value:                327.0000

 Extended solver steps:                 4

 Total solver iterations:              25

 

                 Variable           Value

                OPEN( P1)        1.000000

                OPEN( P3)        1.000000

             VOL( P1, C1)        15.00000

             VOL( P1, C2)        17.00000

             VOL( P1, C4)        3.000000

             VOL( P3, C3)        22.00000

             VOL( P3, C4)        9.000000

Solution: CAPLOC

Total costs are minimized at 327 by opening plants 1 and 3. From plant 1, we ship 15, 17, and 3 units respectively to customers 1, 2, and 4. From plant 3, we ship 22 units to customer 3, and 9 units to customer 4.