Below you can see the front view size comparison of Sony A9 and Nikon D4. Sony A9 is clearly the smaller of the two cameras. Its body is 33mm narrower, 61mm shorter and 28mm thinner than Nikon D4.
Sony A9 vs Nikon D4 - Front View
Sony A9 vs Nikon D4 - Top View
The chart below depicts how two cameras compare with average dimensions of the cameras in their class.
Camera Size Comparison
Select Two Cameras for Size Comparison