Panasonic GH4 has a 16.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor and features Venus Engine IX processor. On the other hand, Panasonic GX8 has a 20.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor and features Venus Engine processor.
Below you can see the GH4 and GX8 sensor size comparison.
Panasonic GH4 and Panasonic GX8 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 Panasonic GH4 has 26% larger pixel area compared to Panasonic GX8, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.