Panasonic ZS40 has a 18.0MP 1/2.3" (6.17 x 4.55 mm ) sized CMOS sensor and features Venus Engine processor. On the other hand, Panasonic ZS50 has a 12.0MP 1/2.3" (6.17 x 4.55 mm ) sized CMOS sensor . Panasonic ZS40's sensor provides 6MP more than Panasonic ZS50'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.
Below you can see the ZS40 and ZS50 sensor size comparison.
Panasonic ZS40 and Panasonic ZS50 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 ZS50 has 49% larger pixel area (2.34µm2 vs 1.56µm2) compared to Panasonic ZS40, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.