Canon 1D MIV has a 16.0MP APS-H (27.9 x 18.6 mm ) sized CMOS sensor and features Digic 4 processor. On the other hand, Canon 1D MIII has a 10.0MP APS-H (28.7 x 18.7 mm ) sized CMOS sensor . Canon 1D MIV's sensor provides 6MP more than Canon 1D MIII'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 1D MIV and 1D MIII sensor size comparison.
Canon 1D MIV and Canon 1D MIII have almost the same sensor size so none of them has any significant advantage over other in providing control over depth of field when used with same focal length and aperture.