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 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 Nikon 1 J4 has 13% larger pixel area (6.36µm2 vs 5.62µm2) compared to Nikon 1 J5, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.