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, Nikon D3100 has a 14.0MP APS-C (23.1 x 15.4 mm ) sized CMOS sensor and features Expeed 2 processor.
Olympus E-M10 IV's sensor provides 6MP more than Nikon D3100'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 D3100 sensor size comparison.
As seen above, Nikon D3100 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.