Newer generations of Optio S4
Thanks for the replies. Although I plan to upgrade later to a prosumer dslr, I still want to get the latest Optio S that uses my existing battery , cases etc. Can I assume that if a later version supports the same battery dl-i8 that the form factor will be the same? The Amazon.com site shows the following models compatable with my battery:
S6, S4i, S5i, S5z, WPi & WP
All of the above cameras are models that are at least seven years old and contain old technology sensors (all six megapixels or less CCD sensors), small low resolution LCD displays, and older electronics and user interfaces and I doubt that you will be able to find any of them available retail as new, thus will not have warranty service or indeed any spare parts available. In addition, the WP series do not share the same exact body shape, so you would only be able to share the same battery and charger.
It seems that you are so desperate to get a camera that shares the same battery as your S4 that you are willing to forgo all the improvements in camera technology that seven years brings.
Now, it seems to me that you are not looking at carrying both your S4 and whatever your new camera might be, so it would seem that you are "holding yourself back" just in order to save the expense of buying a new spare battery for your new camera at perhaps about $50 and perhaps a close fit case for an additional $20 as well as having a spare charger. If you bought your S4 new when it came out in 2003, you likely paid about $400 for it, which would be about equivalent to $700 now, and battery life is probably not generally better than a couple of hundred images per battery charge.
If it were me, I would forgo case and battery compatibility and buy the best dust and water proof and shock proof camera for my use I could find of whatever brand for about $400 and twice the battery life of your Optio S4, also getting the benefits of a new camera with warranty, newer better sensor with higher resolution, higher optical zoom, better ergonomics as to display and user interface, etcetera, while having a battery life of up to about twice as many images as your S4. It is true that these cameras (other than perhaps the Pentax WG2) may be a little bigger than your S4, but does that really matter?