I've seen several posts regarding this in the past and it seems that I have finally figured it out. If you are on an Att prepaid plan with 3g data, you can still access 4g speeds by manually changing your apn settings under the Phone app. Under preferences and accounts change manual settings to on, and change the Internet Apn to ATT HSPA+, leave the username and password blank. This will allow you to activate HSPA+ speeds even on a prepaid account. I have the 25 dollar add on for 500mb and I was seeing speeds of close to 8mbits down and 2 to 3 up. Before I made this change I could only see about 1mbit down. I also never saw the H+ symbol before I made this change.