I have updated the firmware for my Nikon D4 camera.
Nikon promises "Increased focus accuracy is achieved when a lens and teleconverter with a combined maximum aperture of f/8 are used."
and who wouldn't want that?
Before the update, I could not adjust the rear focusing with my 200-400mm lens in combination with the TC-20EIII tele converter. Now it seems I can! But I am getting inconsistent results using the FocusTune software and the LensAlign. I suspect it has to do with misalignment of the test shots. I am awaiting my Manfrotto Super Clamp (under way from Greenland). With that and a telescopic support I will stabilize the camera and lens using two attachments points. I will then redo the tests.
Someone reported on improved focus with D4 in combination with large aperture lenses after the firmware upgrade. I will still have to check on that.