all I know is, from this dpreview page...,
the S40, S4i, and the S4 have the same MP and optical zoom, while the plain S has less MP.
so I am guessing all the three S4 share the same lense and differ in features.
EDIT
oh, I just checked,
the S4i is much more expensive than the S40.
s4i has extra 16 SD card, larger LCD screen, rechargable batteries, one more ISO rating at 50, yet being lighter and smaller????
the S40, S4i, and the S4 have the same MP and optical zoom, while the plain S has less MP.
so I am guessing all the three S4 share the same lense and differ in features.
EDIT
oh, I just checked,
the S4i is much more expensive than the S40.
s4i has extra 16 SD card, larger LCD screen, rechargable batteries, one more ISO rating at 50, yet being lighter and smaller????