The Nikon D3 was a breakthrough camera, offering noise levels and ISO settings that hadn’t ever been simultaneously approached. Two years later, the D3s raised the bar somewhat, offering better ergonomics for the live view feature, and providing slightly better high ISO performance. The difference between the D4 and the D3s is larger than the difference between the D3s and its predecessor, but nowhere near the difference between the D2h and the D3.
The D4 offers D3s noise levels at slightly higher resolution. The economics on the D4 are improved. Pick almost any button on the camera, hold it down, and use the two wheels on the upper right of the camera body – the one on the front operated by your index finger, and the one on the back by your thumb – and you can quickly and accurately make changes without having to access the menu system. This allows the number of buttons and dials to be reduced, and makes the camera easy to learn, since there’s now one control paradigm that’s (mostly) consistently followed.
The D4 buffer is twice the size of the D3s, which had a buffer of twice the size of the D3. This means that, using lossless compression, you can shoot about 60 images at the fastest speed before the buffer fills up if you use a fast CompactFlash card. In the material that Nikon issued at the announcement of the D4, they talked about hundred images before the buffer filled using the XQD card. Since the XQD card that ships with the camera is, at 125 MB/s only slightly faster than a 100 MB/s CompactFlash card, I suspect that you have to use either a yet unannounced, faster XQD card or lossy compression, or both, to achieve the hundred image number.
The D4 still has the beautiful smooth image quality of the D3 series.
The D4 is lighter than its predecessor, but not by much. I understand the video is improved, but I’m not interested in that feature, and I’ve never used it.
My net. The D4 is a very nice camera, and is perfectly suited to my intended use. If you have a D3 or D3s that you’re happy with, I wouldn’t suggest rushing right out to upgrade.