The problem with varying the aperture is that the DOF will change as well. When I bracket, for exposure not DOF, I change the shutter speed. That keeps the DOF the same on all shots.
I calculate what I think is the correct DOF that will get me the right focus range, then stop down one more stop for good measure. But the bracketing will be done with the shutter. Of course, there are no half shutter speeds, only full stops. You can get 1/3 or 1/2 stops changing aperture. But then the DOF changes. I've done this regularly with medium format shots as it's easy and cheap/ I haven't done bracketing with 4x5 though, except maybe one extra shot on the flip side of the same film holder.
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