In zinc blende (ZnS) structure, the co-ordination number of Zn2+ ion is:

we know cations are occupied in the voids and anions are present in the spheres.In zinc blende (ZnS), the sulphide ions are anions and are face centered.It means it has a coordination number 4 while zinc are cation ions which are present in the tetrahedral voids which are alternate.We know that the coordination number of zinc is 4 and sulphide ion in ZnS is face centered so its coordination number is also 4.Thus, the radius ratio rate of the cation to that of anion is 4:4 Zn2+