Start climbing up the wall, then jump on the stone and reach the nearby ledge. Use the stones to continue the climb and get a hold of the ledge to start moving right. Move up and jump on the white mechanism to break the chain and bring some light to the chamber.
Jump below and approach the lit wall. Use it to start climbing, then turn left. Jump through the stone and continue moving left to reach a new mechanism. Jump on it to bring more light to the hall.
Descend from the structure. Look to your right to find a column that can be used to continue the descent. The encountered ledge can be used to reach a large chain found on your left. Jump the chain and use it to reach a new mechanism.
You've managed to regain your former self. Jump on the nearby block to reveal a staircase leading to a shiny portcullis - use them to end chapter 4.