4. Xiaoming formed a sequence using the four digits $2, 0, 1, 6$ (continuously adding these four digits in this order to the end of the number): $2,20,201,2016,20162,201620,2016201,20162016,201620162, \ldots$. In this sequence, the number of prime numbers is $\qquad$.
【Analysis】Only the first 2 is a prime number, all subsequent numbers are not prime, so there is 1 prime number.