Summary: The Nikon D4 camera body is well suited for photographing very fast wildlife on the run. The below photos of a cheetah running a straight 100 meter track were taken at Continuous High Speed shutter release, a shutter speed of 1/1000 of a second, an aperture of f/4, focal lengths of 300mm and 400mm, ISO values between 1250 and 1800, Center Weighted Metering, and Auto White Balance.
Summary: Love it. Gets used daily. Can't really complain at all. I use Nikon D90, D700, D800 and D4. I use primarily Nikon Glass (1.4 50mm, 2.8 14-24mm, 24-70mm and 70-200mm) with the D4, and have zero issues.
Summary: This camera is amazing. No other way to say it. I've had it for a year now after shooting my D3 for years. I wanted a camera that could should cinema quality video. I feel like this camera can take pictures in situations where one would think it's impossible. My favorite lens to use with this camera is the Nikon 50mm 1.4. This lens is so amazingly sharp. Walking around the streets of Chicago it can literally take pictures in the dark.
Summary: I've been waiting a long time for a D4. I've rented them in the past and couldn't wait to actually own one. I purchased mine used with 34k actuations, 3 batteries, extra CF cards, original box and accessories. I was able to contact and speak to the owner and get more pictures of the condition and ask questions about it's history. This was instrumental in my decision to purchase used. It arrived a 10 out of 10 in perfect condition at 2/3rds the price of new.
Summary: Given the price differences between the D800 and the D4, I was seriously upgrading from my D700 to the D800. I found from friends who have the D800 that the D800 is a wonderful landscape and studio camera with probably the best video capabilities of all Nikon cameras. However, the D800 is a bear to use for event and street photography. And forget about using it for sports.