1. **Problem:** If the product of two numbers is $ab$, find the increase in the product when the first number is increased by 4. 2. **Formula and Explanation:** The original product is $ab$. When the first number $a$ is increased by 4, the new product is $(a+4)b$. 3. **Intermediate Work:** $$\text{Increase} = (a+4)b - ab = ab + 4b - ab = 4b$$ 4. **Answer:** The increase in the product is $4b$. This corresponds to option (a) 4b.