Canon 5D MIII has a 22.0MP Full frame (36 x 24 mm ) sized CMOS sensor and features Digic 5+ processor. On the other hand, Canon 5D MIV has a 30.0MP Full frame (36 x 24 mm ) sized CMOS sensor and features Digic 6+ 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 MIII and 5D MIV sensor size comparison.
Canon 5D MIII and Canon 5D MIV 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 compared to Canon 5D MIV, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.
DxOMark Sensor Scores
DxOMark is a benchmark that scientifically assesses image quality of camera sensors. It 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. Canon 5D MIII and Canon 5D MIV sensors have been tested by DxO and the results show that 5D MIV has a better overall score of 91, 10 points higher compared to 5D MIII's score of 81.