What is the special product form of the expression 9x^8-4?

We have ${a}^{2} - {b}^{2} = \left(a + b\right) \left(a - b\right)$ for any $a , b \in \mathbb{R}$.
Thus $9 {x}^{8} - 4 = \left(3 {x}^{4} + 2\right) \left(3 {x}^{4} - 2\right)$