I ended up purchasing the 21 SEER 4 ton IQ Drive. It was installed this weekend. So far it's working great. It got up to 91 deg and 90% humidity out today and the unit had no problem keeping up. At the heat of the day it ramped up to about 75% of nominal capacity for a short period (based on the system information I read off the thermostat). It seems to run a lot at its lowest setting (25% of capacity).
The unit is very, very quite, when it's running at it's lowest setting, I have to open the closet door to hear the air handler. The outside unit can not be heard from inside the house. Even outside the house, you almost have to walk up next to it to know if it's on.
I am anxious to see how much my electric bill will drop.
Thanks to everyone who gave me advice on this. I really appreciate it.