Three coaxial cylindrical sheets are present in free space with radius 2, 4 and 6 m. Each surface has uniform surface charge densities of $2 n C / m^{2}$, $- 4 n C / m^{2}$ and $6 n C / m^{2}$ respectively. Magnitude of electric flux density (in $n C / m^{2}$) for Gaussian cylindrical surface with 4 < r < 6 will be ________.