What does the "4G" that appears on my iPhone 4S's AT&T network indicator mean?
Nomenclature
The terms 2G, 3G and 4G do not refer to a specific wireless standard, but a a whole bunch of different standards:

There has been controversy on what standards should be referred to as 4G. When Apple introduced the iPhone 4S, they followed other companies in referring to HSDPA as a 4G standard. HSPA+ is the successor to HSDPA.

DC-HSPA+
Technically, HSDPA and HSPA+ are 3G wireless standards. However, HSPA+ has received technological upgrades which increased the theoretical throughput to become similar to that of LTE.
If 4G were to indicate speed rather than a wireless standard, it's legitimate to refer to the upgraded HSPA+ as 4G. However, this improved HSPA+ should be referred to as DC-HSPA+, not merely HSPA+.