If a, start subscript, 1, end subscript, equals, 3a
1
​
=3 and a, start subscript, n, end subscript, equals, left bracket, a, start subscript, n, minus, 1, end subscript, right bracket, squared, plus, 1a
n
​
=(a
n−1
​
)
2
+1 then find the value of a, start subscript, 4, end subscripta
4
​
.