4. (6 points) If the numerator of a fraction is reduced by $20 \%$, and the denominator is increased by $28 \%$, then the new fraction is reduced by $\qquad$ $\%$ compared to the original fraction.
【Answer】Solution: Let the original fraction be $\frac{\mathrm{a}}{\mathrm{b}}$,
then the new fraction is $\frac{\mathrm{a} \times(1-20\%)}{\mathrm{b} \times(1+28\%)}=\frac{\mathrm{a}}{\mathrm{b}} \times \frac{5}{8}$, so the new fraction is $\frac{5}{8}$ of the original fraction, $\left(1-\frac{5}{8}\right) \div 1=\frac{3}{8}=37.5\%$.
Therefore, the answer is: 37.5 .