3. Let $f(x)$ be a monotonic function defined on $(0,+\infty)$. If for any $x \in(0,+\infty)$, we have $f\left[f(x)-2 \log _{2} x\right]=4$, then the solution set of the inequality $f(x)<6$ is $\qquad$.
Rong Lun $\{x \mid 0<x<4\}$.