The key to using SFR's prepaid voice and data on your iPhone 4 is so simple. Reset the network settings.
For some reason, on the iPhone4 doesn't pick up the proper settings off the get go. I'm not sure why not, but now after two visits to the SFR store, I've figured out with their very capable techs that "resetting the network settings" seems to release whatever congests the iPhone.
You need to of course not only top up for minutes and have credit on your phone, but you also need to have the "illimite Internet and email" package.