Composite numbers are numbers that have more than 2 factors. With that being said, your answer would be 2.
4 is a composite number: 1, 2, 4
6 is a composite number: 1, 2, 3, 6
8 is a composite number: 1, 2, 4, 8
2 is a prime number: 1 and 2
2 is a prime number, not a composite number.