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, Olympus E-420 has a 10.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor and features TruePic III processor.
Olympus E-M10 IV's sensor provides 10MP more than Olympus E-420'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 E-420 sensor size comparison.
Olympus E-M10 IV and Olympus E-420 have almost the same sensor size so none of them has any significant advantage over other in providing control over depth of field when used with same focal length and aperture.