Canon 5D MIV has a 30.0MP Full frame (36 x 24 mm ) sized CMOS sensor and features Digic 6+ processor. On the other hand, Canon 5D MIII has a 22.0MP Full frame (36 x 24 mm ) sized CMOS sensor and features Digic 5+ processor.
Canon 5D MIV's sensor provides 8MP more than Canon 5D MIII'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 5D MIV and 5D MIII sensor size comparison.
Canon 5D MIV and Canon 5D MIII 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 5D MIII has 36% larger pixel area (39.06µm2 vs 28.69µm2) compared to Canon 5D MIV, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.
DxOMark Sensor Scores
Both Canon 5D MIV and Canon 5D MIII sensors have been tested by DxoMark. DxoMark scores camera sensors for color depth (DXO Portrait), dynamic range (DXO Landscape) and low-light sensitivity (DXO Sports), and also gives them an overall score. Of the two cameras that we are comparing, 5D MIV has scored 91, 10 points higher than 5D MIII.