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, Panasonic ZS15 has a 12.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized CMOS sensor .
Olympus E-M10 IV's sensor provides 8MP more than Panasonic ZS15'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 ZS15 sensor size comparison.
As seen above, Olympus E-M10 IV has a 8.1x Larger sensor area than Panasonic ZS15. 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.