I have a ATT PayGo SIM on my 4s and currently can only do voice calls (no data) as I have not sign up with a data plan, but the 4G icon lights up. I imagine this is solely whether the service is available at your towers and not limited on what kind of data plan you got. The carrier shouldn't care, they charge you by data volume not necessarily how fast can you get at the data.
The 4G logo that you see is what AT&T calls their hspa+
It's not LTE 4G.
what about for verizon prepaid