Nikon D6 has a
21.0MP Full frame (35.9 x 23.9 mm ) sized CMOS sensor and features Expeed 6 processor. On the other hand, Nikon D4 has a
16.0MP Full frame (36 x 23.9 mm ) sized CMOS sensor and features Expeed 3 processor.
Nikon D6's sensor provides 5MP more than Nikon D4's sensor, which gives a significant advantage in real life. You can print your images larger or crop more freely.
On the other hand, please keep in mind that Max sensor resolution is not the only determinant of resolving power. Factors such as the optical elements, low pass filter, pixel size and sensor technology also affects the final resolution of the captured image.
Below you can see the D6 and D4 sensor size comparison.
Sensor Size and Resolution Comparison image of Nikon D6 and Nikon D4 Cameras
Nikon D6 and Nikon D4 have almost the same sensor size, so neither of them has any significant advantage over the other in terms of providing control over depth of field when used with the same focal length and aperture.
Sensor size sets the circle of confusion, so it drives how much depth of field each
of these bodies gives you at a given aperture. You can calculate it exactly for the
Nikon D6
or the
Nikon D4 ,
including hyperfocal distance.