Network has nothing to do with it. Neither the phone nor the operating system is going to have any effect on the network. The iP4S is capable of faster speeds because it can access faster network signals. iOS 5 has nothing to do with this.
LoL dude! NO there isn't! Not unless u jailbreak... iOS 5 is out and you cant get Facetime so no. iP4S is 3.5G but that irrelevant... I'm trying to tell you that you can only facetime on wifi because AT&T was worried about network congestion... So the 4 didn't allow factime unless it was on wifi. The 4S nobody knows for sure yet but my guess is the same, you will have to jailbreak or be on wifi