I have both the D4, and D800. For landscapes and portraits, I love the D800 and its megapixels, but beyond that, in so many ways, the D4 is the much superior camera with so many important features not found on the D800 that it's obvious why it's more expensive by a lot. Here's just a few of the meaningful differences of the D4 vs. D800 already knowing the D800 has many more megapixels.
Built in vertical grip vs. no grip
XQD card vs. SD card
11fps vs. 4fps
Built in ability to use the EN-EL18 vs. EN-EL15 battery giving more than twice as good battery life in the D4, though you can use the better battery with the D800 if you use the vertical grip accessory with it.
Superior high ISO/low noise ratio for the D4
Significantly better build quality and ruggedness in the D4
One can capture more seconds of images at 11fps on the D4 for sports photography, for example, than the D800 can do at 4fps due to its cache.
The D4 has significantly superior button assignment flexibility
Those differences make it easy to understand the price difference. The D800 is a wonderful specialty camera, but the D4 is a professional workhorse, in my opinion.