Graph Coloring · Easy problem

Two servers $S_1, S_2$ each connect directly to all four clients $C_1, C_2, C_3, C_4$ (and $S_1, S_2$ have no link to each other, nor do the clients to each other). Links meeting at the same machine must use different wavelengths. Using König's edge-coloring theorem for this kind of network, what is the fewest wavelengths needed for all $8$ links?