If $x - \dfrac{1}{x} = 4$, $x^4 + \dfrac{1}{x^4} = ?$

From $x-\frac1x=4$, squaring gives $x^2+\frac1{x^2}=16+2=18$. Squaring again gives $x^4+\frac1{x^4}=18^2-2=322$.