Originally posted by ElJamoquio Don't know what the FPS and A/F differences are.
The D4 does 11 fps vs the D800's 4 or 6 and the D600's 5.5, so it's a significant step above.
Although it shares the same Sports/high-ISO score as the D600/D800, the D4 can also crank it's ISO up to 204800, whereas the D600/800 max out at 25600, so the D4 probably has some extra circuitry in there to make the high-ISO look a bit better.
The AF is near identical between D4/D800 (51 point, 16 cross types etc), but the D4 can feed higher fps through the AF system, which may produce better tracking results and so on.
All of this of course highlights the big job ahead of Pentax. Just changing the sensor size won't really impress anyone unless there are other tech smarts included in the product.