Among the following cations, the number of cations which will give characteristic precipitate in their identification tests with $\mathrm{K}_4\left[\mathrm{Fe}(\mathrm{CN})_6\right]$ is $\underline{\hspace{2cm}}$. $$\mathrm{Cu}^{2+}, \mathrm{Fe}^{3+}, \mathrm{Ba}^{2+}, \mathrm{Ca}^{2+}, \mathrm{NH}_4^{+}, \mathrm{Mg}^{2+}, \mathrm{Zn}^{2+}$$