Find the Exact Value of the Limit of the Sequence $a _ { 1 } = \sqrt { 4 } , a _ { n + 1 } = \sqrt { 4 + a _ { n } }$

Find the exact value of the limit of the sequence defined by $a _ { 1 } = \sqrt { 4 } , a _ { n + 1 } = \sqrt { 4 + a _ { n } }$ .