Sigma Quattro H has a 45.0MP APS-H (26.6 x 17.9 mm ) sized CMOS (Foveon X3) sensor and features Dual TRUE III processor. On the other hand, Canon 1D MIV has a 16.0MP APS-H (27.9 x 18.6 mm ) sized CMOS sensor and features Digic 4 processor. Sigma Quattro H's sensor provides 29MP more than Canon 1D MIV'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 Quattro H and 1D MIV sensor size comparison.
Sigma Quattro H and Canon 1D MIV have almost the same sensor size so none of them has any significant advantage over other in this area.