The sum of $4 + 5$ is $9$.

Would you like more details or have any questions? Here are 5 questions you might want to ask:

1. What are the basic properties of addition?
2. How do you perform addition with larger numbers?
3. Can you explain the commutative property of addition?
4. How is addition used in algebraic expressions?
5. Can you explain how to add fractions or decimals?

Tip: Practicing mental arithmetic with simple addition problems can help improve your overall math skills and speed in calculations.