\[= \sqrt{\left( 3 + \sqrt{a} \right)^{2}} - \sqrt{\left( 1 - \sqrt{a} \right)^{2}} =\]
\[= 3 + \sqrt{a} - |1 - \sqrt{a}|\]
\[При\ a \geq 1:\]
\[3 + \sqrt{a} - \left( \sqrt{a} - 1 \right) =\]
\[= 3 + \sqrt{a} - \sqrt{a} + 1 = 4.\]
\[При\ a < 1:\]
\[3 + \sqrt{a} - \left( 1 - \sqrt{a} \right) =\]
\[= 3 + \sqrt{a} - 1 + \sqrt{a} = 2 + 2\sqrt{a}.\]