I agree with dmward, almost 100 percent.
I typically would lock in my camera settings and let ETTL do its thing, but that is just my personal preference in situations where lighting is fairly consistent across the area I'm shooting. If ambient lighting is inconsistent I'd be more likely to let the camera make some decisions.
You will be surprised what bounce flash can provide with high ceilings when set to 105mm "zoom" and with the camera at ISO 6400.
To add to the f/4 thing, this kind of documentary shooting needs the larger depth of field. Staying away form longer tele focal lengths helps here too, although I still try to stick to 50mm or higher.