If a, start subscript, 1, end subscript, equals, 9a
1
​
=9 and a, start subscript, n, end subscript, equals, a, start subscript, n, minus, 1, end subscript, plus, 1a
n
​
=a
n−1
​
+1 then find the value of a, start subscript, 4, end subscripta
4
​
.