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 GH5S has a 10.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor and features Venus Engine 10 processor. Panasonic GH4's sensor provides 6MP more than Panasonic GH5S's sensor, which gives a significant advantage in real life. You can print your images larger or crop more freely.
On the other hand, please keep in mind that Max sensor resolution is not the only determinant of resolving power. Factors such as the optical elements, low pass filter, pixel size and sensor technology also affects the final resolution of the captured image.
Another difference between these two cameras is that Panasonic GH5S'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 GH4 and GH5S sensor size comparison.
Panasonic GH4 and Panasonic GH5S 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 GH5S has 56% larger pixel area (22.14µm2 vs 14.12µm2) compared to Panasonic GH4, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.