4. Simplify $\sqrt{13+4 \sqrt{3}}+\sqrt{13-4 \sqrt{3}}$.
Answer: $4 \sqrt{3}$
Solution: Let $x=\sqrt{13+4 \sqrt{3}}+\sqrt{13-4 \sqrt{3}}$; then $x^{2}=48$. Since $x$ is clearly positive, we take $x=\sqrt{48}=4 \sqrt{3}$