$f\left(x\right)$ and $g\left(x\right)$ are linear functions such that for all $x,f\left(g\left(x\right)\right)$ and $g\left(f\left(x\right)\right)$ are identify functions,if $f\left(0\right)=4,g\left(5\right)=17$ and $f \left(136\right) = 4k$ . Then  the value of $k$