Just got back from my first real shooting with the D4 down at Surf Cup in San Diego (Youth Soccer). I had my D7100 as a second body. Whenever I would switch to the D7100 for closer in shots, I would start getting worried that something was wrong because the AF seemed so slow and the shutter felt delayed as I was shooting.
Before using the D4 I would never have guessed it was so many times better (faster, sharper, more accurate, etc) than my D7100 as that was working great for me. Now it I am thinking I need a D3 as a second body instead.