$$4\sqrt{8}$$
Factor $8=2^{2}\times 2$. Rewrite the square root of the product $\sqrt{2^{2}\times 2}$ as the product of square roots $\sqrt{2^{2}}\sqrt{2}$. Take the square root of $2^{2}$.$$4\times 2\sqrt{2}$$
Multiply $4$ and $2$ to get $8$.$$8\sqrt{2}$$