Olympus E-400 has a
10.0MP Four Thirds (17.3 x 13 mm ) sized CCD sensor . On the other hand, Olympus E-M5 II has a
16.0MP Four Thirds (17.3 x 13 mm ) sized MOS sensor and features TruePic VII processor.
Olympus E-M5 II's sensor provides 6MP more than Olympus E-400'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-400 and E-M5 Mark II sensor size comparison.
Olympus E-400 and Olympus E-M5 II have the same sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture.