30. $y=\sqrt{x-4}$.
Solution. Note that this function only makes sense if the expression under the root is greater than or equal to zero. If the expression under the root is negative, then $y$ is an imaginary number. Therefore, $x-4 \geqslant 0$ or $x \geqslant 4$. Thus, this function is defined only if $x \geqslant 4$, i.e., $x \in [4, \infty)$ (Fig. 65).