Almost at the end of level 4, when I finished beating up those green monsters, I found a lever in one of the corridors where the creatures were enclosed in... flipping it does not seem to do anything. Does anyone know what its effect is?
Apparently, that lever re-activates the portals in the four corners of the main room you were in. I'm too far down to double check the source though.
(Source: http://www.gamebanshee.com/legendofgrimrock/walkthrough/level4.php, section 3)
Edit: Just checked it. It does exactly that.