You know you're in for a treat when Nikon decides to upgrade the very top of its camera range, the single-digit DSLR flagship. Taking over from the D3s is the D4, a $6,000 camera that gleefully upgrades just about every spec from its predecessor while also weighing less and lasting longer on a smaller battery.
Handling of the new camera isn't all that dramatically different from the D3s. A few of the keys have played a game of musical chairs, but you're still looking at more or less the same layout. Nikon has added a pair of joysticks for manipulating your focus point while shooting or moving around an image when reviewing, plus there's now a movie mode toggle framing the Live View button. The biggest change in the D4 is in the way Nikon has duplicated right-hand controls for shooting in portrait orientation — that is to say, the buttons available to you in landscape mode are in exactly the same position when you flip the camera into portrait. I only spent a short time with Nikon's new DSLR, but it's quite obvious how that would benefit those who switch orientation often.
The most immediately appreciable upgrade over the D3s for me was the new shutter release button, which is now slanted down at a 35-degree angle. This brings it closer to Canon's way of building shutter keys, and although copying the competition isn't something Nikon will likely want to be accused of, there's no denying the new design feels better and more natural. Nikon itself admits that the change was made in response to demand from pro photographers.
The rest of the D4's operation is extremely similar to the D3s: the whole camera is built to withstand a nuclear apocalypse, the right-hand grip is extremely comfortable, and the weight balance feels great. Although it's markedly bulkier and heavier than "regular" DSLRs, the D4 feels reassuringly so. I'm actually of the opinion that the larger bodies of such full-frame cameras are more ergonomic to use than the more compact ones of entry-level DSLRs like the D5100. There's more to hold onto and, obviously, the designers put in a lot more thought into how they'll work during prolonged use.
Two other massive advantages for the D4 over more affordable cameras are the viewfinder and speed of operation. The mere stat that this viewfinder offers you 100 percent coverage doesn't convey the sheer luxury and size of the thing -- there's no squinting required here, it's basically like looking through a window. As to speed, the D4 includes the new Expeed 3 processor that debuted on the 1 Series, which makes for even quicker autofocus and an utterly instantaneous shutter release. Honestly, don't pick this camera up if you don't want to hate your regular shooter.
It's worth noting that a lot of these undeniable strengths for the D4 were already available on the D3s. The real upgrades from Nikon will be felt over the long-term and once photographers start to use the camera for more sophisticated purposes. What I can say for now is that Nikon has done a good job building on the fantastic foundation it laid down with D3s in its brand new D4.
See a comparison of the D4 against the D3s, D3x, Canon EOS-1D X, EOS 5D Mark II, and Sony Alpha A900 right here.