Coordination numbers of ${Zn}^{2+}\text{ and }{S}^{2-}$in the crystal structure of wurtzite are 

Wurtzite has face centred cubic structure in which each ${\mathrm{Zn}}^{2+}$ion is attached to four S^2- ions and each S^2- ion remains in contact with four ${\mathrm{Zn}}^{2+}$ions. Hence coordination number of each ion is 4.