Since the momentum vector is $\langle 0,0,4\rangle \mathrm{kg} \cdot \mathrm{m} / \mathrm{s}$, the magnitude is simply the value of the momentum along the z-axis, which is $4 \mathrm{kg} \cdot \mathrm{m} / \mathrm{s}$.
So, the magnitude of the momentum is
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