Below you can see the front view size comparison of Nikon D3 and Nikon D4.
Weight is another important factor especially when deciding on a camera that you want to carry with you all day.
Nikon D3 is 40g lighter than the Nikon D4 but we don't think this will make a significant difference.
Also keep in mind that body weight is not the only deciding factor when comparing two interchangeable camera bodies, you have to also take into account the lenses that you will be using with these bodies. Since both Nikon D3 and Nikon D4 have the same Full frame sized sensor, their lenses for a similar focal length and aperture will be similar in size and weight.