Curves 1 and 2 correspond to similar materials while curves 3 and 4 represent different materials, since the value of stopping potential for the pair of curves 1 and 2 and 3 and 4 are the same. For given frequency of the incident radiation the stopping potential is independent of its intensity.
So, the pairs of curves (1, 3) and (2, 4) correspond to different materials but same intensity of incident radiation.