We know, the radius of curvature of the common surface is given by,

$$r = \left| {{{{r_1}\,.\,{r_2}} \over {{r_1} - {r_2}}}} \right| = \left| {{{2\,.\,4} \over {2 - 4}}} \right| = \left| {{8 \over { - 2}}} \right| = 4$$

$$ \Rightarrow r = 4\,cm$$