For two soap bubbles in contact, the excess pressure inside each bubble is \(\Delta P = \frac{4T}{r}\) where \(T\) is surface tension. Let bubble 1 have radius \(r_1=2\,\text{cm}\) (smaller, higher pressure) and bubble 2 have radius \(r_2=4\,\text{cm}\). The pressure difference across the common surface \(=P_1-P_2=\frac{4T}{r_1}-\frac{4T}{r_2}=4T\!\left(\frac{1}{2}-\frac{1}{4}\right)=T\). This pressure difference is also given by \(\frac{4T}{R}\) where \(R\) is the radius of curvature of the common surface. So \(\frac{4T}{R}=T\Rightarrow R=4\,\text{cm}\). The common surface bulges into the larger bubble and has radius of curvature \(R=4\,\text{cm}\).