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 5DS R has a 51.0MP Full frame
(36 x 24 mm ) sized CMOS sensor and features Dual DIGIC 6 processor.
Canon 5DS R's sensor provides 21MP more than Canon 5D MIV'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.
One other difference between these two cameras that is worth mentioning is that Canon 5DS R's sensor doesn't have an anti-alias (Low-Pass) filter. Removing anti-alias filter increases the sharpness and level of detail but at the same time, it increases the chance of moiré occurring in certain scenes.
Below you can see the 5D MIV and 5DS R sensor size comparison.
Canon 5D MIV and Canon 5DS R 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. On the other hand, since Canon 5D MIV has 67% larger pixel area compared to Canon 5DS R, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.
DxOMark Sensor Scores
Both Canon 5D MIV and Canon 5DS R 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, 5 points higher than 5DS R.
Canon 5D MIV
Canon 5DS R