The Space Gates

I have a start sensor (S1) that detects the initial material on a conveyor to start the program. GATE 1 will close to stop material, once material is stopped and sensor (S4) detects material GATE 2 will close. Once GATE 2 is closed and sensor (S3) detect material GATE 3 will close. Once GATE 3 closes and sensor (S2) detects material GATE 4 will close. When (S5) and (S6) have material present all gates stay closed. When (S5) is empty GATE 1 opens to allow material to move. Once (S6) has material and (S5) is empty and GATE 1 is open then GATE 2 opens. Once (S6) has material and (S5) is empty and GATE 1 and GATE 2 are open then GATE 3 opens. Once (S6) has material and (S5) is empty and GATE 1 and GATE 2 and GATE 3 are open GATE 1 will close and GATE 4 opens to let material move forward. Cycle repeats until all sensors are empty then goes into wait mode.