>I am lucky enough to have both. >I find the d800 is the main camera I grab as I go out the >door just to walk around and shoot. It rewards careful >technique and low iso- I like 800 or below. >D4 is for wildlife and sports. Although I do not consider >myself a spray and pray shooter- the 4fps of the d800 is too >slow for me in these situations. Low light, fast shutter >speed, iso over 1600 is when I grab the D4. >If I could only have one I would definitely choose.....
I just got the D4 to add to my D800, and I echo some of this, however, I remain unconvinced that the D4 does better at high ISO.
I just shot about 3000 frames in the last three days - rodeo, soccer and volleyball. I found myself almost all the time using the D4 (I had both with each time). For speed in all regards the D4 was much better, the 10fps is amazing, and the buffer never gets full (I used a slow CF card and it was never a problem). On the downside, the D800 has spoiled me -- I frame shots now too loose knowing I can crop, and the D4 won't hold up to as much of a crop, especially at high ISO. Maybe it's not that it is worse at high ISO, but for the same portion of the frame having more pixels means noise is less visible.
At over 1000 images in the heat today at the beach the battery finally showed one bar gone -- it lasts a LONG time. If you have a grip for the D800, remember the D4 batteries can be used in it, allowing you to only have one type of battery.
Bottom line: You'll be very happy with the combination.