1 - 0:27 (as you enter the room with the pillar, imagine you are facing the 12-o'clock position. Navigate to the 2-o'clock-ish position, and crouch to swim into the water)
2 - 0:32 (pick up the armor)
3 - 0:50 (jump into the dark alcove)
4 - 2:00 (instead of going straight forward - towards the normal exit, which takes you directly to the last level, Hung Lo's Palace - turn around and carefully jump on the ledges, and make your way towards the secret exit - which takes you to a secret level, Prison Camp)