Canon 5D MIV has a 30.0MP Full frame (36 x 24 mm ) sized CMOS sensor and features Digic 6+ processor. On the other hand, Sigma DP3 Merrill has a 15.0MP APS-C (24 x 16 mm ) sized CMOS (Foveon X3) sensor and features Dual TRUE II engine processor.
Canon 5D MIV's sensor provides 15MP more than Sigma DP3 Merrill'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 5D MIV and DP3 Merrill sensor size comparison.
As seen above, Canon 5D MIV has a 2.3x Larger sensor area than Sigma DP3 Merrill. Larger sensors give photographer more control on the depth of field and blurry background compared to smaller sensor when shot in same focal length and aperture.