Factor $18=3^{2}\times 2$. Rewrite the square root of the product $\sqrt{3^{2}\times 2}$ as the product of square roots $\sqrt{3^{2}}\sqrt{2}$. Take the square root of $3^{2}$.
$$\frac{4}{3\sqrt{2}}$$
Rationalize the denominator of $\frac{4}{3\sqrt{2}}$ by multiplying numerator and denominator by $\sqrt{2}$.
