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⁺