There are probably more experienced cinematographers than I that could give you advise, but it sounds like by setting the camera at 400 ASA, you are using the camera's light meter. It's all realative. The camera can only open the aperature so wide, so even if you set the film speed slower, you only expose the film so much. By pushing the film during the processing to say 800 ASA, you might have more latitude. Are you using any lights? The light meter will tell you everything. Use the manual mode on the camera.
Paul