Olympus TG-4 has a 16.0MP 1/2.3 (6.17 x 4.55 mm ) sized BSI-CMOS sensor and features TruePic VII processor. On the other hand, Olympus TG-5 has a 12.0MP 1/2.3 (6.17 x 4.55 mm ) sized BSI-CMOS sensor and features TruePic VIII processor.
Below you can see the TG-4 and TG-5 sensor size comparison.
Olympus TG-4 and Olympus TG-5 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 Olympus TG-5 has 32% larger pixel area (2.34µm2 vs 1.76µm2) compared to Olympus TG-4, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.