Sony H400 has a 20.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized CCD sensor and features Bionz(R) processor. On the other hand, Canon SX40 HS has a 12.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized BSI-CMOS sensor .
Sony H400's sensor provides 8MP more than Canon SX40 HS'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 H400 and SX40 HS sensor size comparison.
Sony H400 and Canon SX40 HS have sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture. On the other hand, since Canon SX40 HS has 65% larger pixel area (2.34µm2 vs 1.41µm2) compared to Sony H400, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.