Take a look at the photos on this website
1. click on the thumbnail to get the image to appear at the right
2. move cursor to right of image and a blue box with an i appears, click on the i for info including the ISO at which the image was taken
3. compare the different ISO images at X3 large (above the i in the blue box) and a photo will more than fill your monitor. This is about a double page spread size.
4. now vote as to the highest ISO that you think could be used "in the real world" as a full page or double page spread in a magazine.
I think 25,400 ISO images from the D4 are usable full page or double page. What do you think?