Nikon 1 J4 has a
18.0MP 1' (13.2 x 8.8 mm ) sized CMOS sensor and features Expeed 4 processor. On the other hand, Nikon 1 J5 has a
21.0MP 1' (13.2 x 8.8 mm ) sized BSI-CMOS sensor and features Expeed 5A processor.
One other difference between these two cameras that is worth mentioning is that Nikon 1 J5'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 J5 sensor size comparison.
Nikon 1 J4 and Nikon 1 J5 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 Nikon 1 J4 has 13% larger pixel area compared to Nikon 1 J5, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.