Both Nikon 1 J4 and Leica M9-P have 18.0 MP resolution sensors but Nikon 1 J4's sensor is 1' (13.2 x 8.8 mm ) and Leica M9-P's sensor is Full frame (36 x 24 mm ).
Since Leica M9-P's has a larger sensor area with the same resolution, this means that it also has a larger pixel area hence better light collecting capacity for a given aperture compared to Nikon 1 J4.
One other difference between these two cameras that is worth mentioning is that Leica M9-P'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 1 J4 and M9-P sensor size comparison.
As seen above, Leica M9-P has a 7.4x Larger sensor area than Nikon 1 J4. Larger sensors give photographer more control on the depth of field and blurry background compared to smaller sensor when shot in same focal length and aperture.