We start with a state for which the last input is 0. From there, we need four consecutive 1’s to get a 1 output or another 0. Thus, on additional 0’s, we loop back to state A. On a 1, we go to B; on a second 1, we go to C; and on a third 1, we go to D. In D, additional 1’s produce a 1 output; 0’s return the system to state A.