Calculus needed here?

Algebra Level 4

$\large f(x) =\sqrt [ 3 ]{ 1-x } +\sqrt [ 3 ]{ x+3 }, \,\,x\in \mathbb{R}$

Suppose $L$ is the maximum value of the function $f$ . Find the value of $\lfloor 10000L\rfloor$ .

Notation: $\lfloor \cdot \rfloor$ denotes the floor function.