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, Sony A500 has a 12.0MP APS-C (23.5 x 15.6 mm ) sized CMOS sensor and features Bionz processor.
Olympus E-M10 IV's sensor provides 8MP more than Sony A500'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 A500 sensor size comparison.
Sensor Size and Resolution Comparison image of Olympus E-M10 IV and Sony A500 Cameras
As seen above, Sony A500 has a 1.6x 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.