Nikon D750 has a 24.0MP Full frame (35.9 x 24 mm ) sized CMOS sensor and features Expeed 4 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 D750's sensor provides 8MP 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 D750 and D4 sensor size comparison.
Nikon D750 and Nikon D4 have almost the same sensor size so none of them has any significant advantage over other in this area.