3. Given for any $x_{1}, x_{2}, \cdots, x_{2020} \in[0,4]$,

the equation
$$
\left|x-x_{1}\right|+\left|x-x_{2}\right|+\cdots+\left|x-x_{2020}\right|=2020 a
$$

has at least one root in the interval $[0,4]$. Then $a$ equals
$\qquad$ .