Granted, I have yet to hold one and only found out about their existence today, but the Df and D4 are almost apples and oranges. The D4 is geared pretty much solely for the professional market. And it is priced thusly.
The Df has the same sensor but has a focus more on the advanced amateur with a film camera background. I do a fair amount of commercial work but would not spring the money for a D4. But the sensor's are the same so you are getting equivalent IQ for considerably different prices. I have all AI/AIS lenses and never see me getting an AF lens. I just don't see a need. I have 14 AI/AIS Nikkors from 16-600mm. Why switch? All are professional quality and fast for their focal lengths.
And don't get caught up in that 16MP vs 24MP or 36MP furor. Unless you plan on making MURAL sized prints, the differences are not worth the mention.