Which of the following functions is not a p.d.f. (probability density function) of a continuous random variable $x$?

$F_1$ given by

$\begin{array}{c}f\left(x\right)=\left\{\begin{array}{ll}{e}^{-x}& \mathrm{if} 0<x<\infty \\ 0& \mathrm{otherwise}\end{array}\right.\\ \end{array}$

$F_2$ given by

$\begin{array}{c}f\left(x\right)=\left\{\begin{array}{ll}\frac{1}{4}\times \frac{1}{\sqrt{x}}& \mathrm{if} 0<x<4\\ 0& \mathrm{otherwise}\end{array}\right.\\ \end{array}$

$F_3$ given by

$\begin{array}{c}f\left(x\right)=\left\{\begin{array}{ll}6x\left(1-x\right)& \mathrm{if} 0<x<1\\ 0& \mathrm{otherwise}\end{array}\right.\\ \end{array}$

$F_4$ given by

$\begin{array}{c}f\left(x\right)=\left\{\begin{array}{ll}\frac{x}{2}& \mathrm{if} -2<x<2\\ 0& \mathrm{otherwise}\end{array}\right.\\ \end{array}$.