What is the sum of 4 and 5?

The sum of 4 and 5 is calculated by simply adding these two numbers together. In mathematical terms, you take the number 4 and add 5 to it. The operation can be viewed as counting up from 4: starting at 4, counting up five steps (5, 6, 7, 8, 9) leads you to the number 9. Thus, the correct answer, representing the sum of 4 and 5, is 9. Understanding basic addition is fundamental for various applications in mathematics, making it essential for anyone learning arithmetic.