From a purely practical perspective this doesn't sound possible to me so you should be fine. Assuming a consistent 3G signal and an uplink speed of 1Mbps (that's being optimistic) it would take just over 9 hours of full utilization to send 4GB of data. It is categorically not possible in a 15min period.

You won't have to but your defense against AT&T if it ever happened would start with making a request for the logs showing the data sessions, servers accessed, etc.