4. Given that $\alpha-l-\beta$ is a right dihedral angle, $A \in \alpha, B \in \beta, A, B$ are not on $l$, and let the angles formed by $AB$ with $\alpha$ and $\beta$ be $\theta_{1}$ and $\theta_{2}$, respectively, then the range of $\theta_{1}+\theta_{2}$ is $\qquad$ -
$\left(0, \frac{\pi}{2}\right)$;