Comparisons between Olympus Cameras

I'm having a pickle of a time trying to understand the difference between some of Olympus' mid-range cameras. What's the difference between a c-3040, c-4000, d-40, & c-4040?

I know the mp is different between some, but someone once told me that the c-3040 is better than the c-4000 for underwater photography? I'm trying to make heads or tails of this situation, but not much luck so far...

maybe check out www.dpreview.com to see the difference.