First let me say this is not a review of either camera, and I am not telling anyone to spend lots and lots of money to buy either one of these cameras but, if you do, you won’t be disappointed.
The Nikon D800: I’ve had mine now for a few months. I traded in my D5100 which I had for about a year. The majority of the work I do is landscape photography and this is where the D800 shines. It’ a 36.3 Megapixel full frame camera and it makes HUGE files. I have a fairly new laptop that I run Photomatix Pro, Lightroom and Photoshop on, and the thing goes into overdrive mode processing some of my images. The quality of files produced by the D800 is fabulous. The detail is breathtaking, and the fun you can have processing them last for hours.
The Nikon D4: I needed this camera for shooting action sports. Football mainly so far. Fast frames per second is what’s needed, (10fps for the D4) and the D4 will shoot for 10-20 seconds straight without filling up the buffer even in raw format. The high ISO capabilities of this camera are the best I have ever seen. With correct exposure metering I have no fears going even beyond 6400. This is a professional camera, with a professional pricetag. The D4 shoots full frame,16.2 megapixel images all day and never complains. It is a workhorse.
Comparison: Each camera has specific functions. Both are great cameras. Either can be used to do what the other was designed for, but when you use them for their raison d’etre they clearly outshine each other. In closing, remembera great photographer is not made by the camera. Anyone can make great images on cameras costing far less then these.