$$Z_{2}\times Z_{2}=(a+bi)(c+4i)$$
$\left\{\begin{matrix}a=-\frac{-Z_{2}^{2}+ibc-4b}{c+4i}\text{, }&c\neq -4i\\a\in \mathrm{C}\text{, }&Z_{2}=0\text{ and }c=-4i\end{matrix}\right.$
$Z_{2}=-\sqrt{a+ib}\sqrt{c+4i}$
$Z_{2}=\sqrt{a+ib}\sqrt{c+4i}$
