I read about the "darkening" of scenes in patch 4 release notes, it never bothered me so far, but now I got into the spider's tunnels under the well, and I have a lot of trouble seeing the corridors. Even using my PC's with darkvision I find it too dark. I'm slightly vision impaired, so this could be the reason why the effect is so big for me. I had no trouble at all during my previous visit, so I'm pretty sure the game has darkened this.
I searched if there was an option to restore the lighting of the previous patch level, but didn't find it.