در تابع $f\left( x \right)=\left\{ \begin{matrix}
\sqrt{2x+1}\,\,\,\,\,\,\,\,\,\,\,\,\,\,;\,\,\,\,x\ge 2 \\
3x-1\,\,\,\,\,\,\,\,\,\,\,\,\,\,;\,\,\,\,x\lt 2 \\
\end{matrix} \right.$، اگر $f\left( 4 \right)+2f\left( \frac{2}{3} \right)=f\left( a \right)$ باشد، $a$ کدام است؟