In the given arrangement, the slits $S_1$ and $S_2$ are fixed at small separation $d$ while the slits $S_3$ and $S_4$ have variable separation $z$. The screen is fixed and the slits are symmetric with respect to the centre of the screen. A parallel beam of light is incident as shown in figure.



The minimum value of separation $z$ so that the intensity at $O$ is zero is