There is indeed a fourth switch.
Check the bottom-right part of the curch. Hit ALT near the back wall and you should see a key. This opens the gate at the top-left. The switch is behind the painting on the wall.
If you found the above switch, then maybe it's this one:
In the top-right there's a painting on the bottom wall. It's kinda hard to see, you may need to tilt your camera.
