If the surface areas of two spheres are in the ratio $4:9$, then the ratio of their volumes will be:

The surface area ($A$) of a sphere is proportional to the square of its radius ($r^2$):
$$\frac{A_1}{A_2}={\left(\frac{r_1}{r_2}\right)}^2=\frac{4}{9}$$

Taking the square root on both sides gives the ratio of their radii:

$$\frac{r_1}{r_2}=\sqrt{\frac{4}{9}}=\frac{2}{3}$$

The volume ($V$) of a sphere is proportional to the cube of its radius ($r^3$):

$$\frac{V_1}{V_2}={\left(\frac{r_1}{r_2}\right)}^3={\left(\frac{2}{3}\right)}^3=\frac{8}{27}$$

Thus, the ratio of their volumes is $8:27$.