4. Choose any two numbers from $2,4,6,7,8,11,12,13$ to form a fraction. Then, there are $\qquad$ irreducible fractions among these fractions.
4. 36 .

Among 7, 11, 13, choose one number and among $2, 4, 6, 8, 12$, choose one number to form a reduced fraction, there are $2 \mathrm{C}_{3}^{1} \mathrm{C}_{5}^{1}$ $=30$ kinds; among $7, 11, 13$, choose two numbers to form a reduced fraction, there are $\mathrm{A}_{3}^{2}=6$ kinds.
There are a total of 36 different reduced fractions.