The answer is $1,400
Here is the work:
10,000 X 4/100 X 4 years =$1,600 Interest Holly has to pay on 4%, 4-year loan.
10,000 X 5/100 X 6 years =$3,000 Interest Holly has to pay on 5%, 6-year loan.
$3,000 - $1,600 =$1,400 Difference in interest between the two loan choices.