Although I do think its a nice gesture to make a documentary commemorating the 20th anniversary of Diana's tragic death, I really don't see it drawing the interest of a lot of American people. Too much time has passed to draw the attention of people that most likely have long forgotten what happened that night in Paris or even know of Diana and her life outside of the War of the Waleses.
Perhaps four hours is a bit too much even over two consecutive nights. I'm not sure if I'm in the majority of American TV viewers but our weekly prime time viewing here is mostly directed to favorite shows we watch every week. I don't see me watching this documentary. Perhaps if it was on a cable channel that rebroadcasts their shows at different time periods, I would.
I hope I'm wrong on this and its just my opinion.