No. 16 said:
Why don't you just share your knowledge?
The battery chemistries are different between 3GS and 4G. This is public knowledge and printed on the battery for ease in recycling. The 4G uses the more common LI-Ion while 3GS and 3G use LI-Poly. There was one other subtle change in battery between 3G and 3GS, but it is very unlikely that anyone would care. I work with batteries for a living and I would never advise anyone regardless of what it is to drain any rechargable battery to 0 intentionally for many reasons. The thinking on doing such a thing was fine for 1990s based electronics and chemistries, but the battery pack itself is much more than a holding tank now. There's actually more computing power just in the battery itself than many early cell phones. That being said the chemistry theory would make one think that you could do this with the 3rd generation without much, if any, risk. And I wouldn't freak out if you accidentally cause a full drain on a 4.