sequence

I think it is 13. In an Arithmetic Sequence the difference between one term and the next is a constant.

So if we imagine the first number is 1 and then the third is 9, we have these three numbers 1, 5, 9...

We see that there is a jump of 4 every time, so the sequence becomes...

1, 5, 9, 13, 17, 21 ....

See: https://www.mathsisfun.com/algebra/sequences-sums-arithmetic.html

Let the first term  = n

Let the third  =  n + 2d

So

n (n + 2d)  =  9

We have two  different (positive) factors that multiply to 9  →  1  and  9  

So  if   n =  1     and  d = 4   then

1 ( 1 + 8)  = 9

1 (9)  = 9

So the 4th term is    1 + 4(4 -1)  =  1 + 12  =  13