I'm not quite sure where you are. I can only think of two keys in Ch4. One is specifically for the sewer door. The other is for a solid door at the top of a tower at the end of a battlement but that gains you a secret and is not necessary for progress.
I think you may be in front of a portcullis after a hard fight along "the headway" (battlements). If you look into the room and to the right, through the portcullis, you should see a highlighted lever. Shoot it to open the portcullis.
If I've got the wrong place, please post a screenie and I'll be happy to help further.
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