We can use the Euclidean Algorithm. \(\begin{align*} &\text{gcd}\,(a^2+9a+24,a+4) \\ &\qquad=\text{gcd}\,(a^2+9a+24-(a+5)(a+4),a+4)\\ &\qquad=\text{gcd}\,(a^2+9a+24-(a^2+9a+20),a+4)\\ &\qquad=\text{gcd}\,(4,a+4). \end{align*}\) Since \(4\) is a factor of \(a\) and thus \(a+4\) , the greatest common divisor is \(\boxed{4}\).