Nikon 1 J5 has a 21.0MP 1" (13.2 x 8.8 mm ) sized BSI-CMOS sensor and features Expeed 5A processor. On the other hand, Nikon 1 J4 has a 18.0MP 1" (13.2 x 8.8 mm ) sized CMOS sensor and features Expeed 4 processor.
Another difference between these two cameras is that Nikon 1 J5's sensor lacks 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 moire occurring in certain scenes.
Below you can see the J5 and 1 J4 sensor size comparison.
Nikon 1 J5 and Nikon 1 J4 sensors have exactly the same size so none of them has any advantage over other in this area.