I ended up upgrading to the iPhone 5 and discovered that with the upgrade, AT&T will switch you to their new AT&T LTE or "Long Term Evolution" Cellular Service. This service is being currently being aggressively rolled out to new Cities in the U.S. The first thing I noticed that the Cellular service is much better than the 4G, except in some areas such as inside certain building. In one particular case, I was unable to call from a Conference Room where I had previously made many phone calls. So in this case, the only way I could make a phone call (I had only 1 bar using the AT&T LTE Service) was to use the manual procedures below to disable AT&T. Apparently, there are some phone using other services that don't allow this option, but the iPhone 5 with iOS 6 has this option. Below are those instructions. Unfortunately, to re-enable LTE, you need to power off and power on your iPhone.
Finally, for those non-techies, AT&T LTE is not "Lite". AT&T says that it is 10X Faster than 3G.
Instructions to Disable AT&T LTE
1. Setting -> General -> Cellular -> Enable LTE: Off
2. Your phone should switch to 4G
Instructions to Re-enable AT&T LTE
1. Settings -> General -> Cellular -> Enable LTE: On
2. Power your iPhone Off, then Power On
(otherwise, your the data only will switch to LTE, your cellular connection will not change back to LTE)
AT&T LTE Coverage