Answer
$\pm r^2$
Work Step by Step
Use the fact that $\sqrt{a}=a^\frac{1}{2}$ along with the fact that $(a^b)^n=a^{bn}$ to get $$\sqrt{r^4}=r^{\frac{4}{2}}=r^2$$ However, $(-r^2)(-r^2)=r^4$ also. So, $\pm r^2$ is the result.
You can help us out by revising, improving and updating this answer.
Update this answerAfter you claim an answer you’ll have 24 hours to send in a draft. An editor will review the submission and either publish your submission or provide feedback.