`4)\sqrt{8+2\sqrt{15}}-\sqrt{8-2\sqrt{15}}`
`=\sqrt{3+2\sqrt{3}\sqrt{5}+5}-\sqrt{3-2\sqrt{3}\sqrt{5}+5}`
`=\sqrt{(\sqrt{3}+\sqrt{5})^2}-\sqrt{(\sqrt{3}-\sqrt{5})^2}`
`=|\sqrt{3}+\sqrt{5}|-|\sqrt{3}-\sqrt{5}|`
`=\sqrt{3}+\sqrt{5}-\sqrt{5}+\sqrt{3}=2\sqrt{3}`