If the plane 2x – y + 2z + 3 = 0 has the distances $\frac{1}{3}$ and $\frac{2}{3}$ units from the planes 4x – 2y + 4z + $\lambda$ = 0 and 2x – y + 2z + $\mu$ = 0, respectively, then the maximum value of $\lambda$ + $\mu$ is equal to :