Nikon S4000 has a
12.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized CCD sensor and features Expeed C2 processor. On the other hand, Nikon S7000 has a
16.0MP 1/2.3 (6.17 x 4.55 mm ) sized CMOS sensor .
Below you can see the S4000 and S7000 sensor size comparison.
Nikon S4000 and Nikon S7000 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 S4000 has 32% larger pixel area compared to Nikon S7000, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.