The keyword in the question is sum, which indicates that we need to add. The SAT does drop in these operational terms sometimes, so it is worth knowing the following:
sum means add or plus
difference means subtract or minus
product means multiply or times
quotient means divide or divided by
Here, we are asked for the sum of 4 and 5, and, in keeping with your request, that means that we are looking to interpret four plus five, which equals nine.
Good luck with your studies.