Answer
$1.33\times10^{-8}T$ in the +y direction.
Work Step by Step
For an EM wave, $B_{max}=\frac{E_{max}}{c}=\frac{4.00V/m}{3.00\times10^{8}m/s}=1.33\times10^{-8}T$.
The wave is propagating in the +z direction, which is the direction of $\vec{E} \times \vec{B}$. We know $\vec{E}$ is in the +x direction. The only consistent direction for $\vec{B}$ is the +y direction.
