The smallest composite number is _____.

If we look at the whole numbers, 0,1,2,3,4,5, 6, ..., then the first composite number that we encounter is 4. The number 4 has three factors, 

$4=2\times 2$

$4=1\times 4$

So, here 4 has three factors, 1,2 and 4. So, 4 is the smallest composite number.