Currently, the Galaxy S III sports a pretty capable 8-megapixel shooter. Increasing the megapixel count however is no easy task if Samsung wants to retain the slim body line of its flagship Android phone. Moreover, more megapixels also make it more challenging for a device to perform well in low light conditions due to the simple fact that pixels get smaller and noise gets in easier.
The 13-megapixel sensor is said to have been originally intended for the third-gen Galaxy S and the second Galaxy Note, but never made it to those devices (both feature 8-megapixel shooters). If such a camera arrive in the S IV, it'd capture images with 4208 x 3120 pixel resolution.
source: Samsung Updates via UberGizmo