Fujifilm S8600 has a
16.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized CCD sensor . On the other hand, Fujifilm S4500 has a
14.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized CCD sensor .
Below you can see the S8600 and S4500 sensor size comparison.
Fujifilm S8600 and Fujifilm S4500 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 Fujifilm S4500 has 15% larger pixel area (2.04µm2 vs 1.76µm2) compared to Fujifilm S8600, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.