Olympus E-M10 IV has a 20.0MP Four Thirds (17.4 x 13 mm ) sized CMOS sensor and features TruePic VIII processor. On the other hand, Canon 1D MII has a 8.0MP APS-H (28.7 x 19.1 mm ) sized CMOS sensor .
Olympus E-M10 IV's sensor provides 12MP more than Canon 1D MII'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 E-M10 IV and 1D MII sensor size comparison.
As seen above, Canon 1D MII has a 2.4x Larger sensor area than Olympus E-M10 IV. 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.