1, 1, 2, 3, 4, 5, 8, 13, 21 - which one doesn't belong to this series?

In the given question, we are tasked to find the number which does not belong to the group. The series are 1,1,2,3,4,5,8,13,21. By simply looking on the series, we may opt to add the first number by the second number and the result would be the third number. Like,

1+1=2

1+2=3

2+3=5

3+5=8

5+8=13

8+13=21

As we can see in the pattern, the only number that is in the series which is not in result is the number 4. The series given above except number 4 is called the Fibonacci Series. Thus, the answer to the problem is 4.