the D4 actually as slightly less dynamic range at the lowest ISO than at 200
The drop won't make any difference in real photography but it is interesting and has been noted by other researchers.
Since I display actual data this could simply be within the accuracy of the data. In other words, maybe it's flat rather than sloping.
There are some hints in the data that the camera may actually operate the sensor in a different mode at low ISOs. I participated in a lengthy discussion on the topic at dpreview. Participants included Marianne Oelund and Eric Fossum. No conclusion was reached, we need more data than is currently available.