4. $$ \frac{9m^2}{25a^2} : \frac{18m^5}{35a^5} $$

$$\frac{9m^2}{25a^2} : \frac{18m^5}{35a^5} = \frac{9m^2}{25a^2} \cdot \frac{35a^5}{18m^5} = \frac{9 \cdot 35 \cdot m^2 \cdot a^5}{25 \cdot 18 \cdot a^2 \cdot m^5} = \frac{315 a^5 m^2}{450 a^2 m^5} = \frac{7a^3}{10m^3}$$