4. There are 5 fractions: $\frac{2}{3}, \frac{5}{8}, \frac{15}{23}, \frac{10}{17}, \frac{12}{19}$, if arranged in order of size, which number is in the middle?
4.【Solution】The least common multiple of the numerators is 60, and the 5 given fractions are sequentially equal to:
$\frac{60}{90}, \frac{60}{96}, \frac{60}{92}, \frac{60}{102}, \frac{60}{95}$, comparing the sizes of the denominators, the median fraction is $\frac{60}{95}$, which is $\frac{12}{19}$.