Nikon D4S - Manual (Page 416)

394
U
Compare retouched copies to the original photographs. This
option is only available if the retouch menu is displayed by
pressing the
J and 2 buttons when a copy or original is displayed
full frame.
1
Select a picture.
Select a retouched copy (shown by a
& icon) or a photograph that has
been retouched in full-frame playback
and press the
J and 2 buttons.
2
Select Side-by-side
comparison.
Highlight Side-by-side
comparison and press
J.
Side-by-Side Comparison
J button