No, there's no 4th passageway, but the key is at the end of the 3rd, right around the spot where you would click if there was a 4th passageway....
if you really can't see anything there (try boosting contrast as well), you're going to have trouble once you get the key...lots of dark passageways in the sewer. (Oh, and the key that is in there is not the sewer key, but it will get you the sewer key...).