Dual Nature of Matter and Radiation: JEE Main 2020 Physics question

When radiation of wavelength $\lambda$ is used to illuminate a metallic surface, the stopping potential is V. When the same surface is illuminated with radiation of wavelength 3$\lambda$, the stopping potential is ${V \over 4}$. If the threshold wavelength for the metallic surface is n$\lambda$ then value of n will be __________.