$$\left\{ \begin{matrix} & {{a}_{2k+4}}\equiv {{a}_{2k+3}}-{{a}_{2k+2}}(\bmod 5) \\ & {{a}_{2k+3}}\equiv -{{a}_{2k+2}}(\bmod 5) \\ \end{matrix} \right.\Rightarrow \left\{ \begin{matrix} & {{a}_{2k+4}}\equiv -2{{a}_{2k+2}}(\bmod 5) \\ & {{a}_{2k+3}}\equiv -{{a}_{2k+2}}(\bmod 5) \\ \end{matrix} \right.$$
$$\Rightarrow \left\{ \begin{matrix} & a_{2k+4}^{2}\equiv 4a_{2k+2}^{2}(\bmod 5) \\ & a_{2k+3}^{2}\equiv a_{2k+2}^{2}(\bmod 5) \\ \end{matrix} \right.\Rightarrow a_{2k+4}^{2}\equiv -a_{2k+2}^{2}\equiv -a_{2k+3}^{2}(\bmod 5)\Rightarrow a_{2k+4}^{2}+a_{2k+3}^{2}\equiv 0(\bmod 5)$$