Question: A circular disk is cut into n distinct sectors, each shaped like a piece of pie and all meeting at the center point of the disk. Each sector is to be painted red, green, yellow, or blue in such a way that no two adjacent sectors are painted the same color. Let Sn be the number of ways to paint the disk. Find a recurrence relation for Sk in terms of Sk-1 and Sk-2 for each integer k ≥ 4.