4. There is a math competition problem, the probabilities of A, B, and C solving it alone are $\frac{1}{a} 、 \frac{1}{b} 、 \frac{1}{c}$, respectively, where $a 、 b 、 c$ are all positive integers in the range of 1 to 9. Now A, B, and C are solving this problem independently at the same time. If the probability that exactly one of them solves the problem is $\frac{7}{15}$, then, the probability that none of them solves the problem is $\qquad$