4. A senior high school grade is to select 3 representatives from 5 boys and 2 girls to participate in a mathematics competition (each person has an equal chance of being selected). Given that boy A is selected, the probability that at least one of boy B and girl C is selected is $\qquad$ .
Let $A=$ "at least one of boy B and girl C is selected", then $\bar{A}$ represents removing boy A, and selecting 2 representatives from 4 boys and 2 girls, where boy B and girl C are not selected. Thus, $P(\bar{A})=\frac{C_{3}^{2}+C_{3}^{1}}{C_{6}^{2}}=\frac{2}{5}$, so $P(A)=1-P(\bar{A})=\frac{3}{5}$.