4. In a perfect dielectric medium with the permeability of \(\mu\), there exists an electric field given as \(\mathbf{E} = E_0\cos(\omega t - kz)\hat{a}_x\), where \(E_0\) is the peak value, \(\omega\) is the frequency, k is a constant, and \(\hat{a}_x\) is the unit vector in x direction. What is the magnetic field (H) in the region?

(A)\(\frac{kE_0}{\mu\omega}\cos(\omega t - kz)\hat{a}_y\)

(B)\(\frac{kE_0}{\mu\omega}\cos(\omega t - kz)(-\hat{a}_y)\)

(C)\(\frac{\mu\omega E_0}{k}\cos(\omega t - kz)\hat{a}_y\)

(D)\(\frac{kE_0}{\mu\omega}\sin(\omega t - kz)\hat{a}_y\)

(E)None of the above

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