Answer
$$\theta=\frac{\pi}{2}.$$
Work Step by Step
\begin{align*}
\cos \theta &=\frac{\langle 0,3,1\rangle \cdot \langle 4,0,0\rangle}{\|\langle 0,3,1\rangle\|\|\langle 4,0,0\rangle\|}\\
&=\frac{0}{\sqrt{10}\sqrt{16}}\\
&=0.
\end{align*}
That is $$\theta=\frac{\pi}{2}.$$
