The new Nikon D4 is a logical and incremental update of the current Nikon D3s. Video capabilities have been significantly improved, the sensor has been changed to a higher resolution 16.2MP CMOS and maximum frame rate has been increased to 10fps.
The D4 boasts an ISO range of 100-12,800 (basic) but can be extended to 50-204,800 via custom functions. The previous 1005 pixel metering has been upgraded to 91,000 pixels which now offers face detection(!).
The Nikon D4 will now compete with the new Canon EOS 1D X for the "king of the hill" position among DSLRs.
The Nikon D4 is expected to be available in stores sometime in late February 2012 at a price of around $6000 (note: the Canon 1D X is expect to be in the stores in March and sell for $6800). As of early March 2012 the D4 hasn't yet shipped to stores but should do so soon.
You can check price and order status here: