There are a few great posts on the web on how to speed up your phone. I won’t try to re-create them. I personally like the version from lifehacker and the few links they have related to Battery life and settings
Overall, the idea is to disable anything that is continuously talking or communicating to the web. Basically, notifications and location services.
I have not upgraded to a 4s yet, but a major reason I would make the change is for performance. My 4 is not the fastest thing at the moment. However, it is good enough. I have done a quick race with my wife and her 4s is quite faster.