Answer:
See explanation below.
Explanation:
Let X the random variable that represent the demand for the magazine, the pmf for X is given by:
X       1            2           3          4        5        6      
P(X)  1/15      2/15       3/15     4/15   3/15     2/15
3 magazines
For this case the total spent is 2*3 = $ 6
And the net revenue for this case would be:
$4-$6 = -$2 , X=1 (demand 1)
$4*2-$6 = $2 , X=2 (demand 2)
$4*3-$6 = $6 , X=3 (demand 3)
For the values of X=4,5,6 the net revenue will be $6 since the number of magazines is 3
And the expected value for the net revenue would be:

4 magazines
For this case the total spent is 2*4 = $ 8
And the net revenue for this case would be:
$4-$8 = -$4 , X=1 (demand 1)
$4*2-$8 = $0 , X=2 (demand 2)
$4*3-$8 = $4 , X=3 (demand 3)
$4*4-$8 = $8 , X=4 (demand 4)
For the values of X=5,6 the net revenue will be $8 since the number of magazines is 4
And the expected value for the net revenue would be:

As as we can see we have a higher expected value for the case with 4 magazines.